That's an S12 chassis, and none of them came twin cam, but some came with the 3.0 V6 from the 300ZX. Most of them were CA20E, and some rare ones were turbo, CA18ET. It's about the same as a RWD Celica but heavier. The first thing you need in those cars is suspension, they bodyroll and squat really badly. It's independent suspension though, so not too bad of a car to try on.
I would still recommend an early 535i, but you're on the right track. Any decent fab shop should be able to weld a diff BTW, but remember it's pretty hard on the halfshafts. I think old Nissan pickup diff with the M12 bolts will work in a 200SX, or a Z31 diff too, ask Ostrich about that stuff though.
-Sean
__________________
At a given day and a given circumstance, you think you have a limit. And you then go for this limit, and you touch the limit, and you think "okay, this is the limit". As soon as you touch this limit, something happens in you so that you can go a little bit further. With your mind power, your determination, your instinct and your experience as well, you can fly very high. - Ayrton Senna