A Glock in it's base form is a plenty capable pistol - totally fine for use in competition. The problem is that the ergonomics of the Glock don't necessarily work for everyone or in many instances - people just want to tinker and not leave well enough alone. The list below will be fairly comprehensive in what you can do and there will be stops along the way where you can "get off". I'll finish with my preferred set up used in USPSA carry optics and how I'd adapt it to IDPA. It's always good to qualify the advice you're getting on competition - because it's given freely and often worth what you pay for it. I'm a "Master" classified shooter in USPSA in both Carry Optics and Production and a "Master" in Enhanced Service Pistol in IDPA.
Base Guns
So to be totally clear - a longer barrel doesn't "get" you anything other than a little more velocity and expense when buying the gun if you use dot sights. If you're using iron sights - then the extra barrel DOES get you additional sight radius which makes hitting easier.
For Carry Optics division in USPSA and IDPA I recommend the Glock 17 gen 5 MOS. I prefer the MOS guns because they come with all the plates you'd need to use virtually any optic. If you're new to the game it gives you plenty of options. The plates and screws they provide aren't necessarily the most robust - but I've yet to personally have a failure with five MOS guns I have. Once you're settled on an optic and or a footprint - then getting a Glock 17 gen 5 milled.
I did a video comparing the two and gave the nod to the 34 for optics - but I've since reversed my stance on this. I like the faster cycling slide of the 17 and it draws a bit quicker and more importantly the slide provides better balance than the 34 variant. I started my Carry Optics career with a Gen 4 17 MOS. Gen 5 is a bit advantaged due to ease of maintenance, better trigger, and a flared integral "magwell" on the frame.
For Irons divisions - still the Glock 34 gen 5 MOS is the best gun in the Glock family - the flash sight picture yields best hits with a gun that has the longest sight radius. The heavier slide makes recoil seem a bit milder and will tame 115 and 124 grain loadings a bit better than the 17 will. The 19 doesn't have as much real estate on the frame to get a grip and control recoil - you're far more likely to pinch your palm on a reload with a 19 than a 17/34.
Get a gun wherever you can get the best price on them - but Big Daddy Unlimited may have the best price going as far as magazines are concerned. It's absolutely worth it to sign up just for a month or two and stock up on several magazines.
Magazines
So it's no secret you can save some money getting Pmag GL9 magazines and I would even recommend the PMAG 21 rounders for USPSA Carry Optics practice. The PMAGs are great - and they generally work fine - until they don't. Unfortunately - the PMAGs will have their magazine springs wear out faster than an OEM Glock magazine would.
The OEM Glock magazines are basically bullet proof and pretty affordable as is. They're a bit heavier so they drop free a bit easier than the PMAGs. For matches - I wouldn't risk running PMAGs.
Base pads aren't strictly necessary on Glock magazines in IDPA or USPSA production - but are nice to have - especially if you shoot indoor matches and drop magazines on concrete a lot. Plastic base pads can crack and one day explode (very amusing when this happens!).
Dawson Precision has inexpensive +0 mag bases - but so does Springer Precision
In USPSA you're going to need a +5/+6 base pad. The "Go To" for a lot have been the Taran Tactical base pads and I have used those to decent effect. The magazine springs wear out pretty quick so get a couple extras to keep on hand - especially if you go 23+1 a lot. The Dawson Precision easy off ones are decent but come with a stouter spring so won't do 23+1 until the springs start wearing out. I prefer how the Dawsons come on/off the mag tube as you WILL need to clean your mag tubes if you drop them in the mud / sand. I like the way the Henning Group base pads handle the best though - but you pay for it.
The one thing you have to keep in your head regardless of which direction you go on a base pad in a high cap division in USPSA is down load that magazine by 1. If it's at full capacity on a slide forward reload (typical) then there's a good chance they won't lock in and will fall out and make you look dumb. Ask me how I know. ALWAYS download any magazine you'll be attempting to lock in under the buzzer.
In IDPA - no reason not to have a magwell if you're in ESP or Carry Optics.
Triggers
Triggers are a touchy subject (hah! Get it?). Ostensibly - there is no reason to make any changes to a gen 5 trigger. You can spend a fortune on a Glock trigger and it basically will end up feeling like a more refined Glock trigger. The results are tough to improve on in a meaningful way. That said there are about 3 ways you can improve your trigger - I'll summarize then expand.
Before getting started there are two - maybe four tools to work on a Glock. A 3/32" punch, a Magazine disassembly tool, a nut driver / front sight tool and a sight pusher.
Basic trigger job - use all the same parts that come in the gun but polish the mating surfaces so there is less friction. The trigger pull weight will vary depending on what connector came in your gun. If you're using a 17 then you can count on a 4.5# or so trigger pull with a reasonably firm wall and if you're shooting a 34 then it will probably be closer to 3.75#-4#.
The trigger won't impress anybody at the safe table and your friends who shoot CZ triggers will wonder how you even. Because they "can't even". The upside of this trigger group is that it basically is maintenance free and the pull weight will keep dropping as the over powered striker spring slowly begins to wear out.
Enhanced Trigger Job - The same as the above but you're changing connectors, striker spring, and firing pin block spring (plunger spring). This Ghost kit is pretty typical of what is changed in the gun. The reduced power firing pin spring will make take up more linear and lighter, the connector geometry will affect the firmness of your wall - whether it's more of a hard wall versus a rolling break, and the striker spring will affect how much pull weight is required to finish cocking the striker and releasing it. It will still feel like a glock trigger - but it will be lighter and the springs will wear out quicker. The overtravel can begin to be removed from the trigger at this point depending on the connector you use. There isn't a huge difference between the connectors - some come coated with an antifriction coating and those are a little nicer - but there isn't a big change.
Advanced Trigger job - Basically the same as above - plus you are swapping out a trigger shoe, potentially a trigger bar, as well as potentially the striker itself. The most meaningful change will be a new trigger shoe - or the part of the trigger your finger interfaces with. Depending on how it's shaped it may make the geometry of the pull easier to pull straight to the rear. You can just do the shoe and nothing else potentially and get a lot out of it. If you put an extended /lightened striker in you can get away with using a lighter striker spring weight. You can go bananas with a full drop in kit - but I generally am less and less inclined to think they provide a meaningful change.
The one trigger kit that actually DOES materially change how the Glock trigger functions is the Timney Alpha Glock Trigger that I reviewed previously. It may be worth what they are asking for it - but in my shooting I can't really exploit a difference but I'm sure there is no shortage of people who are believers and swear by it.
Put simply - I wouldn't monkey with a Glock trigger beyond a very light polish - if I could even be bothered to do that. They're not that different and lighter springs means a shorter spring life. At most I'd do a connector and maybe a trigger bar/shoe if I couldn't make peace with the Glock trigger shape. Grip Modification
Glocks don't feel great out of the box. They're blocky - the angle of the grip is weird if you don't understand how to grip a Glock, and the texture isn't particularly great. For competition shooting - I'd figure out if you need a backstrap on the gun. If you're a medium hand size you probably don't. Larger hand sizes might play with how the medium or large backstraps affect your trigger reach. If you are prone to slide bite use the one with a beaver tail.
You can use decal grips - but they wear out pretty quickly and seem to shift if you practice a lot or use it on a really hot day. You can go more permanent with silicone carbide - but that loses traction as it fills up with your dead skin and grime.
The single best "modification" is using some form of grip enhancer before you shoot. Apply it to the heel of your support hand palm that contacts the frame, the inside of the knuckles that grip the front strap of the pistol and backs of your knuckles where the support hand locks onto your grip. You don't want too much traction to where you can't shift the gun for manipulations but grip enhancer is basically free and is basically free hand strength. Use it. Among high level shooters it's not a "do you use this" question it's a "what kind do you use?" question.
The only other thing that I need to reliably drop magazines is the Tango Down Magazine release. This release is brilliant because it moves the actionable part of the mag catch from the front of the button near the guard to the rear of the button - where you're more likely to be pressing it. It's not overly large to where it gets in the way - it just makes the mag catch easier to hit positively. This is a "must have" upgrade on all of my Glocks. I can change nothing else on the gun and I will change this.
And one thing that's overlooked - but is totally worth doing - is a plastic grip insert in the cavity at the heel of the gun. This acts as a funnel to reload the gun if you can't use a magwell - like in USPSA.
Sights
So this is pretty easy - if you're competing with irons - Dawson Precision is the only way to fly for fiber sights - or something similar - Warren Tactical is good too.
If you want to get cowitness sights for your optic - if it's not an RMR /Holosun - then Dawson precision has you covered. The Ameriglo GL-429 sight set is inexpensive and good enough. Eventually - you don't need cowitness irons functionally in competition. They're great training wheels or a stop gap if you shoot strong hand or weak hand only and go nuts.
As far as optics go - the Trijicon SRO is my personal favorite and what I'd consider the best in breed for gamer sights currently going. 5 MOA is the "best". The SRO gets a ghost dot when the angle of the sun is really low and you are shooting into it - you can put a paster over your lens and shoot it as an occluded sight and call it good.
Slightly more common than the SRO and less costly is the Leupold Deltapoint Pro in 6 MOA. Still an expensive optic - but a good window and now a good dot - easy to change battery. Lousy button for adjustment.
The cheapest option that I've been pleasantly surprised by is the Bushnell RXS-250 - it's cheaper than the Holosun 507C everyone loves - the shroud has a better design - the buttons are better - the battery is bigger. It's a great dot that shares the footprint of the DPP but I'd get a couple before they get sued by Trijicon - they're fantastic sights.
Recoil Springs / Guide Rods
This one is something that I've moved around on. There are basically 4 schools of thought - use the OEM ones / weight, use a stainless uncaptured rod, use a recoil reduction system, or use a heavy guide rod - like a tungsten.
Initially - I was team tungsten guide rod. Now - if you don't have a glock yet - and you want to do all this stuff to it - just buy a Sig Legion.
The OEM system is OK.
A stainless guide rod with a 13# spring - maybe a 15# spring depending on load - is OK. The advantage is not recoil reduction - the advantage is a single linear recoil impulse rather than the dual captured spring rate. You can replace the springs with commonly available springs. The balance remains similar to OEM - which is good.
The DPM recoil reduction system does seem to actually do stuff. Makes the gun harder to rack but reduces felt recoil as well. Since this is steel the balance remains good - the spring shave a ridiculous service life - like 80k+ rounds in me talking to their rep. Set it up with the lowest power springs they can give you and forget about it.
The tungsten guide rod does make the gun recover from recoil and settle in faster off transition a bit nicer - at the expense of making the gun more muzzle heavy. Glockstore has ultra heavy guide rods (use code: Humble10 to save 10%). This is great - but does even better when paired with a brass grip plug and magazine base.
My recommendation on recoil springs are they don't matter much. Your grip will get used to the timing of the gun and generally be OK.
Conclusion
So how would I set up my guns if I were to build them now?
USPSA Carry Optics - Glock 17 g 5 direct milled for SRO. Sights deleted. Tango Down magazine catch. Henning group base pads. For IDPA - swap in Dawson magwell and base pads. Light trigger polish. Minimum of 8 magazines for USPSA (4 OEM with base pads / 4 PMAG 21 rounders) - in IDPA minimum of 6 magazines.
IDPA SSP/ USPSA Production - Glock 17 g 5. Dawson Precision competition sights. Tango Down magazine catch. Dawson base pads. IDPA ESP - add in the magwell. Minimum of 10 magazines for USPSA, 6 for IDPA.
The Glock is plenty capable as is. I said 17 on irons for me personally because I am sensitive to the gun's balance and I prefer the balance of the 17 to the 34. For most - I would recommend the 34 if shooting irons - but if that's just too much money - I wouldn't feel too put out. It's not like the 34 is a radically different gun - it's a 17 with a longer slide and barrel. If you have to have the minus connector - you can put one in the 17.
And that's it... There's always more you can do to a competition gun - but I'm not convinced the ROI is really there. Some of the links or codes used above are affiliate links and generate a commission on sales at no additional cost to you.