[greglepore]

I have/had 2 w140's, an s600 currently and a s420 before. Also a 600 r129 and a w124 320 wagon.
The harness issues are a pain. Don't know if they're still available or if you need to fab them now. Other wiring goes too-like the stuff to the folding mirrors etc.
The 320 engine is prone to external head gasket failure, leaks like a valve cover gasket but lower, until it finally fails. 2k-3k at an indy to repair if head needs surfaced and/or you need other cooling system stuff, so tend to it if its leaking.
The seat bladder dissolve into dust and run the vac pump full time if you don't turn off the dial, which cooks the pump. There is a diy fix. Soft close and locks depend on the pump.
And a/c evaporators pre 95 are an issue, dissimilar metal corrosion, dash needs to come out for repair. I've not seen a failure on my cars, but it happens. I used cryoseal prophylactically on mine with good results.
Big heavy cars with lots of rubber in the suspension and drivetrain, all of which needs tended to.
They are worth the trouble though. Very serene driving bank vaults. Analog cars. You can feel the quality.
Prior owner is key. If its been neglected, the catch up will kill you. If its been cared for, they can be dead nuts reliable with normal care.
One thing more-like everything else, parts availability is getting sketchy. MB no longer supports "everything" - some stuff is nla, others only thru the classic center, and parts that are available are often from 3rd party suppliers with what seems to be the norm now with high failure rates.