If I were thinking Porsche, I would be looking at 911 SCs. Light weight, lots of parts available and a smog era engine that should benefit significantly from IT prep. You can find them for your $7k price too if you look.
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My thoughts on the 911SC are little gains to be had with CIS, headers will help some, adjustable cam timing a plus but engine will not see the 25% gains seen elsewhere. The T bar suspension also presents it own set of headaches. Massive oversteer may be a plus though....

My .02 is Boxster or 968 is the P car way to go