I agree with Cartman

15k will buy you a heck of a 951 with room to mod. I was looking at $5k-7k for a running 951 but in need of a complete drivetrain revamp. As it was to be a track only car I wasn't too concerned with body condition, only that it be straight, etc. If you're budget is $15 then you have a significant amount of moola to get wicked fast at the local PCA event.
In a 911, well let's just say it would be a fixer-upper. The best thing to do is go drive a 944 and a 911 then make up your mind...