Agree with Tilbs in as much as the engine and gearbox are likely to be the least of your worries if not abused.
Vacuum powered central locking will fail. Self lubricating plastic impellor wears overloading motor
Dashboard backlighting is high voltage CFL rather than LED and will fail at some point. Quite expensive to fix.
It will rust violently at some point if not treated to anti rust measures earlier in life
Mine has a quiessent drain the on the battery through the same circuit as the radio (suspect something to do with convenience pack but not had time to investigate yet)
Airmatic suspension bags/pumps/level sensor actuators are all fairly common fails...
Steering wheel buttons are poor...
Search W220 encyclopedia for more ...
Still one of my favourite cars and will only get replaced when the W221 becomes cheap