Apart from the usual body rot and chassis as has been said before :-
Check Chassis and box sections for rust and accident repairs
Check for Leaking sunroofs
Check for Water getting in above the rear door.
Check Air suspension, Is it sitting level?
Does the air compressor come on at start-up?
Check for leaks at the bottom of the suspension air bags
Check for Smoke, Look for blue oil smoke, black diesel smoke (overfueling), heavy black diesel smoke (failing turbocharger) or white smoke on starting (water in the bores)
check steering (bushes tend to wear out)
Check for Coolant leaks, Td5 water pumps can leak, causing coolant level to drop, then overheating occurs....
Check ACE (Active Cornering Enhancement) pipes for corrosion. Can be seen behind the off-side front wheel (very costly to put right)
Check all warning lights including abs/hdc/tc lights on the dash come on AND EXTINGUISH once started, if not then there's hub sensor problems