All you really need for a new Glock:
Night Sights (even YEARS from now when the Tritium fads, you'll still have steel sights)
To maintain it forever, you should have on hand (and this depends on how much you shoot it):
* A small fist full of recoil springs. Replaced every 5K rounds or so. (Most folks will never FIRE 5 thousand rounds, so the stock one it comes with will be fine)
* A small fist full of magazines springs. Replace as needed
* One extractor. You'll probably never need this if you load your pistol correctly
* A couple of trigger springs. Again, you'll probably never need these, especially with the new Third Gen Glocks. I shot a Glock for a little over a year with the trigger spring removed, so if you should break one, you can replace it when you get around to it.
* A replacement set of pins. Again, you'll probably never need these. I too have had a broken trigger pin. Found out when I went to detail strip the pistol (100K rounds through it at this point). I was halfway across the country teaching at the time and didn't have a spare. I simply turning the pin 180o degrees, taught one class and took a class. No problem. When I got home I replaced it.
* A few extra OEM magazine