There is nothing wrong with the diesel R class, so long as you know what your buying...I have had ours just under 2 years and 30k miles, it is close to a 100k miles in total. 2 years trouble free, yes a check engine light twice that needed an egt sensor cleaned (not replaced, just serviced so no actual cost, 10 minute job without a hoist or jack).
Fuel economy is astounding, with 17" snows 7L/100kms (30mpg+), with 18" summers 8L/100kms, with 22" summers 9L/100kms. Highway driving.
It starts no matter the temp outside, it has heat no matter the temp outside (1000watt electric grid heater), it cruises at any speed in absolute comfort and outhandles anything "minivan" so much it is crazy, then there is the winter traction true all wheel drive rather then slip and maybe grip minivans...
Yes they are complex vehicles heavy with electronics, lift the carpets and false floors to reveal a half dozen modules/computers/amps etc, separate rear fuse box/sam unit etc. Just buy the "right" vehicle, took me 8 months to find one that was owned not leased and cared for/had zero salt exposure (FLA wintered) and not repainted/full of dents/stains/damage. Rust protection is very evident all through out the vehicle for long term (thick wax everywhere, find that on any minivan!). Watch for vehicles that had the floors/carpets flooded to clean them or flood vehicles, if the sub floors got wet good bye electronics! Same for salty carpets, make sure it did not get down underneath.
Depreciation is extreme on them, don't bother with dealer CPO. Mine was 60%off retail within 3 years, no warranty remaining. Dealers were running 40% off retail, the warranty is not worth ten grand+!!! The condition of the CPO vehicles was not genuine either, they were mostly repainted or had accident claims against them, not exactly free from defects as the CPO states!

My feeling is the dealers are not able to fix them, they just replace parts and parts and parts...you need to find a good mechanic that is equipped with current technology (very important!). Looking at service records really stupid diagnostics/repairs were performed that did nothing to fix the vehicle, like oil leaks from crankcase blowby were diagnosed as rear main seals, replace egt sensors when cleaning them works for not just months but years, silly repairs that cost way more then they should. They don't spend time to diagnose properly, just assume it needs this, and send the customer away with a big fat bill but still not fixed. They really don't understand diesels...

You'll notice the paint is often really sun burnt/full of scratches it is a really really hard clear coat that you cannot easily buff up, major scratches can be hidden but swirl marks and sun damage very hard to fix. The hoods take a serious beating, lots of damage on them when you see under the lights (water spotting, chips, dents from pushing it closed, bird droppings etc)

I love mine, no regrets and am glad I spent as much time seeking out the one I did and not settled on a cheaper one, or one with warranty remaining after getting treated the way I have at the dealer, I don't want anything to do with them so why pay extra to have to deal with their warranty? They just expect you to buy a new one, not fix the old one!

A couple of comments on options (mine is loaded except leather), pano roof is amazing for the passengers, not so much for front seat occupants (too far back) but offers tons of natural light, HID headlights are a must so bright and strong, Airmatic is a great feature to have the adjustable suspension for rough roads or sensitive passengers really really nice feature (but very rare option), DVD entertainment package is pretty sad when you have young kids as the remote only works in the rear seat no way to control from the front seat (for $2000+!). MBtex/Artico is much better then leather for care and cleaning. The grey interior is terrible to keep clean, the carpet stains so easily (ex coffee stains are impossible!) my only gripe about this car, not black inside.