Look at rear door handle for rust, and the edges of the wheel arches - if the car has been always fully serviced by MB do a rust claim to have those fixed (if car under 7 years old, which it should be)
They eat brakes disks & tyres, like most 4WD so check them and make sure they are good or have been replaced recently
Centre armrest gets very worn, replacement is about 100 quid from memory.
Over some years the front suspension sags (I had a ML500, so a bit heavier) and causes the inner edges of the tyres to wear quicker than outer, if thats the case it needs a full geometry check (mine was done at about 5 years old I think, car looked higher afterwards, and I had a siezed trackrod end that needed replacing, all in all it was a few hundred quid)
Officialy COMAND adds about £800 and the family pack (7 seat option) adds about £1000
Xenons are nice, memory seats make life easier if you have multiple drivers.
Ours was very reliable, only had an alternator/battery and a aircon radiator (that got a stone hole in it), and theres not huge amounts of electronics so very easy for an indie to service !
Richard