I think if that car was listed in the Pelican classifieds, it would bring mid 30's pretty quick.
Here's another black C2 that just sold, was listed at $39k, so I assume it sold for something close to that price, which is surprisingly considering all the non-original bits.
http://forums.pelicanparts.com/porsche-cars-sale/825302-1990-black-black-c2-manual-coupe.html
If you can get the car in the OP for $30-35k, you'll be doing just fine. Sounds like a good car that needs just a little differed maintenance.