I think this depends on whether you will need it as a road car to get to the circuit with or if you can put it on a trailer ! If it has to go on the road it always jacks the price up as you will need mot , tax etc. Plus a different set of wheels ,brake pads +fluid etc.. The world is awash with ex one make challenge cars , Fiestas , Clios, stockhatch 205's etc that change hands for just a few grand already race/track prepped , if you try to build one up it costs double the puchase price of an already done car . If your current roadcar is a Sierra it may be better to pick up a caged Sapphire as , if youre anything like me , youve probably got enough spare bits to make a complete car for peanuts ! Rolling shells are available from £500 upwards , you can even stick a tuned pinto in it to save money and still embarass faster cars .