Do some searching on
www.gt40s.com, all your questions have been covered at one point or another, and some, like how much power a Audi transaxle will handle have never been really answered. I do plan to answer that one though!
Power is easy to make. The problem is, you're dealing with a transaxle and not a transmission, so power is hard to contain. That has been covered at length though so I won't go into it again here.
You need to hook up with some GT40 owners and go for some rides or drives. That will help you decide if one is for you or not.
Ron