Scoobies are a hoot on track, but they're always the first car parked up in the paddock on a TD. Apart from a Noble, of course.
Coldel wins this with his VX suggestion. Nothing else comes close if you want a quick track car for that money, BUT be aware that one prang could write it off easily.
Completely biased, but this would be fun:
MR2 mk3 £1500
Suspension £600
Brakes £300
Hardtop £500
Bracing £200
Turbo kit £4000
Seats, harnesses £600
R888s £500
That's £8300, so room to allow for fitting of parts as well if you like. My suggestion includes a set of tyres, too