Cars I'd like to avoid, BMWs. Why? Because it's just too fecking obvious.
I've considered the following:
Mk1/Mk2 MX5 (then upgrade brakes, tyres, susp, seats, strap a turbo on)
Datsun 200SX
Honda S2000
Something with a bike engine in it e.g. Formula 27 etc (ruled out due to maintenance)
MR2
...and that's it

It just looks like a really short list so I thought I'd defer to you motley lot for advice.
The budget is 4-6k including fast road/track set-up mods. I know that's not a massive budget and I'll probably go over but it's a start. I'm not that bothered about paint, interior and consumables. Something cheapesh that has loads of tuning potential, many fast road/track accessories available and an exploitable RWD chassis. I think I may be doing myself a disservice not including BMW, so I'll review that stance upon opinions presented. I have seen BMW setups that look stonking fun for the money. I'm not a BMW hater by any means, like I said, it just looks like the obvious choice.
No AWD or FWD options thanks, it's mid-corner RWD adjustability I crave, thanks.