I think £12k should get you a half decent useable one. With a budget of £12k, you could be looking at ones advertised at around the £13k mark with the view of knocking them down a bit. So a pretty decent 60/70/80,000 miler may not be out of the question at all.

As long as the mechanicals are all good, look out for crunchy gear boxes, smokey turbo's, slipping clutches and head gasket problems at this mileage. Might also be worth looking for one thats had some engine work or a rebuild at that sort of mileage too as most seem to be ready around then.