The diesels avoid alot of engine issues, but there is always a chance of transmission issues, electrical stuff, air springs, etc. I have one and love it, but I know what I bought and am prepared to work on it when needed and have parts suppliers. Most buy them blindly and don't realize how complex the vehicle is and the fact that modern cars are only built for the warranty period and no more. The dealers pretty much dump them once the warranty is up, they are just too complex to keep servicing. If you have a good mechanic or your into it yourself you can have a good long life out of one, it was well put together and easily worked on for the most part (air and fuel filters are a pain!). It is not as well put together as the older S class, but still shares the same engineering and will likely provide a long service life if wanted, few bother and give up once more then one thing breaks (we are a throw away bunch now!).
Be sure to check it out very thoroughly by a specialist, have the computers scanned (with a factory reader, not OBD2 crap-there are dozens of computers). Look for salt build up in the carpets, the majority of the computers are under the carpets in a sub floor compartment and salt can ruin the harnesses etc. Try the tailgate many many times, it should always open and close smoothly, not slam shut or reopen on its own. Test drive and feel how the transmission shifts while driving slow, does it hunt or let the rpms spike when shifting? Check out the suspension very well, it can get expensive but so far seems well built aside from the airbags.