Well, what caliber do you want to shoot.
I don't like 40. I don't see the point. That's just me. Shoot 9mm or 45 imo. Want a revolver, then shoot 357. That's like all those rifle calibers. 7mm, 300 wsm, wtf. They all cost more than 300 win mag. What's wrong with 300 win mag. centerfire rifle, to me, is 223, 308, 300 win mag, 338 lapua and 50 bmg. The rest is not needed. 22 lr for practice ( I don't even shoot that much .22 anyway, but I have 2 pistols and a rifle). 17 hmr is nice for small game hunting because you can't hunt small game with centerfire.
Do you want a full size glock or a smaller glock with an aftermarket barrel? (will cost more, and barrel will be longer than slide). They still look nice imo.
Personally I wanted a 9mm full size glock so I got a glock 17, and I wanted a 1911 45. The 1911 only shot 50 rounds and is sleeping in my safe. But it looks good and it's a collection piece. Well, it costs more to shoot 45 and I don't reload it so I don't really feel like paying 30$ to shoot another 50 rounds.
Could get a 45 glock, but 45 is more expensive to shoot. I want to target shoot for cheap, and 9mm is the best value.
My suggestion is, if it's your first pistol, start with a glock 17 full size 9mm, then see.
Unless money is no issue to you, you will end up shooting the caliber that costs the least most of the time.
I don't regret my glock 17. Good, reliable, fool proof, shoots better than I do. I shoot it most of the time. I also like my beretta 92fs 9mm because it looks so good.
In the usa with CCW, it's another story. I would probably carry a small .45 glock. But here, your pistol will only be used at the range to target. 9mm is cheaper.