The diesels are by far the winner hands down in terms of longevity. There are forum member here with diesels with easily over 200K miles (just breakin' her in)! The hardest part is finding one in good condition. Some previous owners have driven them to the ground (I've stayed away from 10+ yr old Volvos for the same reason).
I can't speak about the 400E and 500E series. The 3.2L 300E is probably the most common and popular of the series. MB trannys aren't problematic as far as I can tell, but the M 103/104 series engines have been historically known for having head gasket failures when the mileage gets in the 100Ks. Mine is at 108K and so far, so good. I figure when it happens, it will be due for some head reconditioning anyway.
Overall, the car has been very reliable. My only hassle has been with the A/C evaporator failure (fixed under an extended warranty), and the engine cooling system (all components replaced with in the past six months), which I attribute more to age.
I have found parts to be reasonably priced, and the car very serviceable by a DIY'er like myself. I have had other daily drivers that have not fared as well, when called upon for daily duty.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car