A diesel at this price has the potential to be problematic. You want one with reasonably high miles and extensive service history - it's better to find a diesel that's done lots of motorway journeys rather than one that's been used around town. I'd be tempted by a Skoda Octavia. They're popular with taxi drivers, which is a sign that they're pretty reliable, but look for signs that it might have been used for private hire. Also consider a Toyota Avensis for the same reason.