He can get a pre-G50 Carrera with 100k+ miles for less than 12k. I did.
If he were interested in building a track only car, I like the idea of a 2100lb 78 SC no sunroof coupe with fiberglass panels and a 3.2 or maybe a 3.6. Or maybe a race prepped 930 with almost 600bhp. But when you say street/track, it sounds like this guy wants to drive it to work. He might even want to [gasp] listen to the radio. The best car in this situation for the money you're talking about is a 84/86 Carrera.
Stock, the 86 is the best of the lot as it has the bigger torsion bars in the rear and bigger sways. With a modified suspension, there is little to no difference between 84-86. I think the 84s still had the trombone oil cooler, so you may want to watch out for those.
And yes, Carrera's do tend to eat alternators, but replacement
should be a 1 hour job. I think David had angered the Porsche DIY gods somehow, and they unfortunately exacted their revenge in a harsh manner.
