Honestly, mags and ammo. There's not much wrong with a G19.
Sights: Any quality sights of your choosing. I like Trijicon tritium sights.
Mags: At least 5-10 FACTORY mags.
Holster: At least 1 quality holster that works well for you (range use, etc.).
Ammo: If you have the first three, shoot the snot out of it until it's seen at least a case (1000rds).
At this point, you will have a pretty good idea of whether or not you want to try different sights, extended mag release or slide lock lever, etc.
Weapon light: Not necessary, but very useful when properly employed. I am using the Streamlight TLR-1S with the strobe feature deactivated. Surefire is also great. If you mount a light, you'll probably need a holster that will allow you to keep the light mounted
Return to the ammo step above.