Trijicon sights and a good holster. Milt Sparks for IWB in leather (their VM2 is the shit!) or perhaps a Blackhawk lvl.2 for OWB in plastic. A good gun belt, don't overlook that. It is akin to a good scope on a good rifle. Don't use a walmart belt to carry. The Galco and Bladetech belts aren't the best, but they are sufficient and will do the job nicely.
Lots and lots of cheap FMJ or plated ammo to practice, a few hundred of the good carry stuff (pick on, Glocks eat 'em all). Shoot it a lot, get familiar with it, practice with it.
As for modding it, you will hear of folks who put all kinds of shit in their Glocks, but Glocks run best with factory parts. Don't do shit to it, just shoot it. You can't upgrade them, you can only be sold on upgrades. Get it? The only aftermarket parts I use are recoil springs and guide rods in my range pistols. And from time to time, those do jam up. Not with factory parts though.
A Glock isn't one of those add on and upgrade type weapons out there and more common today. If it came with real sights, it wouldn't need anything at all done to it.