without a doubt a saxo VTR is your best bet. VTS/106GTi 16v's are slightly more expensive to buy and insure if you can fork out for it, but only slightly faster and got to remember a VTR 8v lump can be bought for like £50-100 and is bullet proof anyway.

http://www.kamracing.co.uk/car-tunin...roen-saxo.html

from that website you can essentially rebuild the entire car with uprated components.
no car i know off is as cheap to modify to such a standard and few cars are as much fun on a tight track for similar money.

most track days are full of them, and people just like you on small budgets.