From what I know about Bader's Ghia (if only he would frequent this site, things would be so much nicer), he just used the 944 ABS sensors at the wheels, and a 993 pump and controller. Since on the 993 most all of the electronic systems are independent, it's not too much of a wiring nightmare.

Conceptually it's pretty simple to retrofit, but finding a car with a very similar weight balance seems trickier. In talking to him (Jochen), he said the tire diameter is irrelevant as well as the actual weights on the corners. So long as the tires are the same height, and the balance is close, you're golden.