No alot to worry about on these really, considering the amount of technology in them, they're pretty good.
Check for missing digits on the dash display (mileage etc.)
Check the tyres for uneven wear (they rumble loudly when driving).
If you're buying from a dealer, insist they replace the rocker cover breather filter as these are prone to blocking causing turbo or even engine damage.
Make sure there are no knocks or squeaks from the suspension.
Operate the air suspension up and down a few times and make sure "air susp. inactive" doesn't show up on the display.
check the leather for wear, especially the outer edge of the drivers seat base. Seat covers for these are about £500 each!
Thats about all i can think of off hand.