Just north of Damascus, where I live, is a long stretch of country land before Ashton and Olney, MD. In the middle of this stretch a man was selling a car on his lawn this summer - his race-ready Buick Grand Nation (88K on the frame, 15K on the motor) with beefed up internals (rods, pistons, etc.) and huge turbo... he said it ran low 12s, I didn't doubt it. The strange part was, the guy didn't seem at all interested in the car or racing or anything - just a very "blah" attitude. Maybe he was a relative or something. His price was $20K firm - my range at the time was $8K tops.

Here are my top 5 in the sub-15,000 category:
1) early-mid 80s Porsche 911 Turbo S. Can be had for under 15K, and with some work they can put out the power of the 996 and GT3 of today. Plus - that classic look.
2) fully decked out Del Sol with turbo B-series, but with no hints on the outside - no stickers, 3-piece rims, anything like that. It would be difficult to dounder 15, but possible... just barely. Seeing as I have a Del Sol now, makes me wish I had 15K to spend ...
3) 1968 Dodge Charger R/T with the 426 Hemi. I wouldn't even mess with it, just stock would be enough. In black, though. Has to be black, just like the car Dennis Hopper drives in the movie Blue Velvet.
4) GMC Typhoon. Very similar to your Syclone, but with a little more room, more passengers, more creature comforts, etc. 4.3L turbo V6 can be easily pushed up above the 400hp range... with some good tires, it could beat most cars out there.
5) One of the early 80s, 1st gen torpedo-shaped Rx-7s (cheap!) and then drop in a 3-rotor Wankel engine with hardy apex seals, supercharged, and painted white. It would push 300hp, in a super light-weight body with a look which nobody would immediately recognize.

lastly, here is a page that deals with dropping huge engines into little cars, like your Fiero plan:
http://members.tripod.com/~grannys/