First, congratulations on your giving in to the Dark Side. It may be ugly, but the Glock 17 is one of if not the best combat pistol ever made. I proudly own one myself. As far as upgrade modifications, I'd say the big one would be to install a set of Meprolite or Trijicon 3-dot night sights. IMO, that is an essential upgrade for any pistol. The nice thing about the Glocks is that they really don't need modifications, they're great as-is. You don't need to install a heavier recoil spring, the stock one is fine for 9mm. It won't hurt the gun if you do, but it's not necessary. One modification some Glock owners make is to install the Glock 3.5 lb trigger connector. This supposedly lightens the trigger pull and helps with some peoples' accuracy. My Glock 17 is second generation and completely stock other than the Meprolite night sights, and the extra-power springs I've installed in all my full-capacity magazines (FML and NFML).
Make sure you only use Glock factory mags however. As with ARs, aftermarket Glock mags are generally junk. Pre-ban G17 mags are relatively plentiful and less expensive than other Glock model mags.
Lastly, enjoy your Glock 17, clean it once in a while, shoot as much +P+ ammo in it as you'd like, and practice, practice, practice. The Glock triggers can take some getting used to.
BMAN