Further to Nicks post I would write a checklist before going to see the car and check everything off against the list, if satisfied buy it.
This list should include but not be limited to:
Start engine from cold to ensure it starts Ok without noises.
Check steering, brakes, lights, suspension.
Check underbody and exhaust for rust.
Check for oil leaks.
Condition of radiator and water pump for leaks.
Check ALL electrical devices such as central locking, sunroof, windows, radio, wipers, washers, vent fan, etc.
Check VIN number against V5 and all MOT certs.
Check VIN number in at least two places, windscreen, drivers floor, slam panel.
Check sellers address is on V5, if not why not?
Check Mileages on ALL paperwork.
Check panel gaps and alignment and colour match.
Check tyre wear is even across the tyres.
Check colour and condition of antifreeze and brake fluid. This is a good indicator to a well serviced car.
Check general interior wear is in accordance with mileage.
HPI check for write off and outstanding finance situation.
If all the above are Ok the car and seller seem good then negotiate a price subject to clear HPI report.