If you buy a W210 E55AMG (I did) make certain it has FMBSH and no missed services or intervals of over 2 years between services. That way you get the benefit of the Mobilio extended warranty which, the way MBUK seems to handle goodwill claims at present, should cover you for the nasty rust issues they have. Mine just went through the goodwill warranty process at seven years old and I got £1,000 of free bodywork repairs.
That aside, yes, they do incur garage bills but if you get a properly maintained example, then about £1,000 pa for service/repairs on around 10-12,000 miles pa (tho can be expensive on tyres).
Lovely engine - never gone wrong except a new MAF sensor at 101,000 miles.