Major things to look for with one of them are
1) make sure all electrics work, from windows, central locking, roof, alarm, all lights etc. if possible 2 working keys (expensive to replace) and the EKA. When the Air-con / heating is on make sure the little book symbol is NOT lit as this shows there are faults if it is.
2) Engine wise they are pretty bullet proof IF looked after properly, some - not all did suffer from porus blocks. Oil changes really should be every 5k. Look for any cylinder head leaks (mine went on the passenger bank at the back of the engine and I'm told its a favourite place for them to go, due to the coolant routing and poor coolant maintenance).
3) As said earlier Air-suspension - Best left on air-bags, the ride is 100% better. If it starts leaning to one side or sagging at the rear, the bags will be perished where they fold over on themselves. (NOT the end of the world, but a great bargaining tool for £££ off if you can DIY. The Arnott ones are the preferred replacement expensive, but have a lifetime warranty.