I think that the 240SX is the most overrated car ever. I don't see what's great about them at all. That said...
- Porsche 944 Turbo S (or, much less likely, a Porsche 968 Turbo S - there's like four of them, and the last one for sale sold for about $45,000, not exactly affordable but still pretty reasonable for an ultra-rare supercar)
- Porsche 924 Carrera GTR (or, more likely, one tuned to GTR status. 450hp out of a 2.5L four-banger is my kind of energy, and you could do it for cheap)
- a DeLorean DMC-12 (there's a famous one around the 'Net with a 911 Carrera 2 6-speed transmission and a tuned twin-rotor out of the 3rd generation RX-7. It's fast. Very fast. Although that kind of work might be a little pricey.)
- Lancia Delta HF Integrale 16V Turbo (there was one in the US on eBay not long ago for about $9,000)
- Alfa Romeo 164 Cloverleaf (ultimate 4WD sleeper if you could get one here, it'd be probably be about $12,000)
- Alfa Romeo GTV-6 Callaway Turbo (rare, but quite fast)
- Audi S6 (the original one from the mid-90s with the I-5 turbo. ultimate sleeper, something like 0-60 in 4.9 seconds with a boost controller, and a rich man's sound system. They go for about $14,000)
- Mercedes Benz 190 E 2.3-16 and 2.5-16 (Benz's answer to the E30 M3, fast, pleasant to drive, and a yardburner with tuning. Cheap too at $6-7,000)
I could probably go on for hours. When I was first looking at cars to get I made a "dream list" of affordable cars 2 pages long. Almost none of them were feasible, haha.