-Stock manifolds aren't necessarily bad...however I would like to provide replacement manifolds that have been v-banded and get away from the 3-bolt flanges. Would the added labor of replacing the manifolds deter you from purchasing?
Should be a selling feature to have a kit that gets rid of the crappy stock rear manifold.

-How complete of a kit is everyone looking for? Are we talking whole 9 yards? (injectors, gaskets, fuel system, etc etc), or just the turbo hardware (hot side/ cold side). Or just offer both?
Offer a complete kit and let people contact you for custom purchases, let them know you're open to it and email you to discuss.

-How much power would the majority of you like to see from your kits?
I doubt you'll get the majority of your buyers looking for 600-700whp. 500whp is likely the magic number. I'd want something that spools quick.

HTH