I'm shopping for a '69 Porsche 912 at the moment. I'll keep this car closer to stock, but I have a feeling it's perhaps still the most under-rated Porsche of all time.
A new Porsche 961 may be on the list when it comes out- waiting to see what they come up with. If it's not my cup of tea I might try to pic up a GT3 RS 4.0 if that's practical...
1969 Porsche 911 w/ 997 GT3 Cup Motor (435hp & 2,100 lbs)