^^^ what he said.
If you are talking about importing one now, then there is no real point, you can get plenty of cars that are already in the UK cheaper. If you are talking about cars already in the country, then do as Andy says and just get the cheapest.
Mods I'd be looking at are decent rubber, new suspension, brakes (braided lines, racing fluid, decent pads and discs), ARBs, seats + harnesses, then engine mods and chasis bracing.