check the service book and see what was done at the previous service, if its close to 40k, check the major service has taken place, also a o7 or 57 plate will need its cambelt and maybe the water pump replaced so try getting one with it, also check the engine model, if it starts with AXX, check the tensioner chain is not making loud ticking noise! be sure the car is a UK car and not an import (this is in the first page of the service book). take it for a ride on the a road or motorway and push it to about 80mph see if it pulls to one side or if wishbones need replacing.... you will know by the feel of the drive! if dsg gearbox, test in sports mode and see the difference.
check the smell if cloth interior, there are some smelly people around! lol
good luck, try going for a 1 owner car then you know its been looked after...