avoid anything without a full service history if you are spending £6k, also a warranty is a must.
Anything with a dual mass flywheel is best avoided, repairs can be £800/900, same with a cam-belt replacement ( unless it fails, which means a new engine)
Rust is not a big issue anymore, but other things are, some cars have headlights that can cost a grand to replace (BMWs hid type) , Renaults can have faulty dashboards, Audi spares are v.expensive, VWs to a lesser extent. Kia are selling very well at the mo, the seven year warranty and the £99 deposit and monthly payment being a great incentive.
For complete peace of mind there is only one car; no insurance worries, no servicing costs, no rip-offs at the garage, no grief over mots, no fall-off-a-cliff depreciation, and that's a company car.