Yes, buying a track-specific car is a good idea. If you overcook it during the day, at least you can get home.
A couple of hundred pounds will get you a geat saloon (Astra, Escort etc), and there are many companies doing tune-up bits fot them to go faster, stop better etc. for about £1000, you can have more fun on track than your Impreza had on the road.
BTW, if the thread goes off towards the Monoposto route, I'll move it to the Racers secton of Ten Tenths, as many racers have details of test days etc.