Miata. Scores low on the fast criteria, but perfect 10s on the fun to drive, pretty reliable, and good handling areas. It is the perfect autocross car. You can pick up a NICE early 1.6 for $4K easily. $7K should get you an early 1.8L as well. Look hard enough, and you might be able to find a '94 R package for very close to $7K.
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Matt
92 SE-R from Heck
"What have you DONE to that thing..." - stranger at Rockingham dragway 5/18/01
"Where's the turbo?" - stranger at the 1st autocross of 2000 where my SE-R debuted
"That thing is a goddamn rocket ship. I'm not lifting for you on the straights anymore!" -Karl Shultz, Rockingham 4/27/01
"What in the hell have you done since I drove it last?" -David Stone, Rockingham 4/27/01