if you get a glock, you need not do anything to it, maybe change the factory plastic sights to steel. but other than that it is good to go. no fancy triggers, no fancy plugs, buffer rings extended mag releases, mag funnels, slide releases (shouldn't be using that anyway).
get ammo, mags, gear to support it's purpose and training.
I am assuming that you are gonna use the gun as a "fighting gun" right?