I think 4K would get you a reasonable RS Turbo with CVH and in need of a little work, 6 and up would get you a mint car. Im not sure how much a properly (ie done nicely) example with a zetec would cost but I would bank on at least 6K for something worth owning, they have gone up in value quite a bit in the last 3-5 years.

The good news is that if you spend over the odds and buy a genuinely mint example the value will never go down. So in 5 years time I wouldn't be surprised if a 6K rs would be over 8k as all the doggy ones fall off the road in into Dagenham heaven.