But you worry about affording the insurance - the running costs on anything vaguely quick will probably be MORE than the cost of an expensive insurance policy. Thats my point.
An £8k Evo requires a service every 4500 miles. These are also not cheap services.
Then you've got to consider reliability. Very fast cars are expensive - cheap ones are cheap becuase they are getting on a bit, and thus reliability will become an issue unless its a TVR in which case reliability was an issue the day it rolled out of the factory gates anyway. When stuff goes bang, which from time to time it will, it will again cost big money to put right.
I'm not trying to put you off, having a car thats reasonably quick is great fun, but make sure you know what its going to cost you - its going to cost you a lot.