I'd say you'd be making an excellent choice if you decide to go with a Glock!
My 17 was my first firearm, I purchased it when I was 21. I love that gun and don't ever plan to give it up! Actually, I hope to someday buy a "brother" for it so that I can have a pair.
Glocks are definitely fun to shoot. The 9mm especially if you pick up a couple of 33 round mags and let them go rapid fire!
heAs far as the price of ammo 9mm is far and away the cheapest, 100 rounds of Winchester White Box target ammo is about $10 at WalMart. I don't really know how much .45ACP costs, but I've been paying about $13 for 50 rounds of target ammo for my .45 GAP. I've found a couple of local stores that are carrying the GAP (such as Gander Mountain), but I usually just order in bulk on the internet anyway. The major defensive rounds are all available for the GAP as well, from Silvertips to Gold Dots. I also have some Hydra Shocks.
No matter which model you get I would recommend getting night sites. I got the factory night sights with my 37 and I love them. I'm now definitely planning to upgrade my other 2 Glocks with better sights.
Bottom line, my advice is to get the .45 GAP if you want the fit of a 9mm with the power of the .45 ACP. If the full size frame of the 17 (and 37) isn't for you then the 38 will most likely be available by the time you're looking to buy. As far as upgrades go, you don't really NEED anything except for the night sights. The 37 comes with an extended slide release already, you could add an extended mag release or fiddle with the trigger if you really wanted to, but Glocks don't need anything, they really are "Shooting Perfection". That being said, there are tons and tons of mods that can be done to personalize a Glock, just take a quick look around the web and you'll see what I mean.
All Glocks are DA only. The factory trigger has a short take-up to a very noticeable "break point" at around 5lbs. I've done some trigger work on my 17 and its down around 3 lbs. Unless I'm shooting them side-by-side I don't really notice the difference between the factory trigger in the 37 and the modified trigger in the 17. I've found that for me Glock triggers are the easiest to learn and to shoot well with. The only other handgun I can shoot as well with is my brother's S&W revolver in SA mode.
Hope this helps!
Later,
Matt (yeah, I'm a Matt too!)