I'm going for a £6.5K (completed) cap here.
1. Renault 5 GT Turbo. The car has a simple 1.4 litre OHV engine dating back to the 50s, a carb but also a turbo. The turbo can be wound up and up and up. Some of them do 120bhp/litre, some do more with intercoollers.
2. Kit cars - you can pick up a few unfinished projects or older builds for this kind of money and rebuild them for very little other than a lot of time and some paint and some cheap parts.
3. Original Mini. Pick up a not too neat but original one (i.e. no bike engines etc.) and add all that classic tuning stuff from the 60s and 70s.
4. Peugeot 309 GTI 1.9. Show it a twisty road and anything, including a Vette or a VW GTI, will
not be able to keep up.
And even it's smaller brother, the 205 GTI 1.9, will struggle.
5. Hmmm - need to think...