At 65,000 miles, that was what was happening on my 99 S420. The engine and drive train were bulletproof, but the heater/air conditioning had to be replaced, the hydraulic vacuums for the doors had to be replaced, and the trunk had stopped functioning again, the S.R.S. light had come on and the quote was $140 to diagnose, and repairs possibly in the thousands.
All stuff that could be fixed, but if you have to pay a mechanic (as I do) then it is thousands of dollars. I turned mine into an Audi S8 with an extended warrranty, and got a much faster, much better handling, more comfortable, better appointed car (2002 model). Quite happy with the changeover.
We then got a Lexus RX330 for my wife, and she couldn't be happier. Intellichoice, Consumer Reports, and so on, give Lexus very high marks for reliability. Much better than modern Mercedes.
I don't think I'll be back to a Mercedes. We'll see how the Audi holds up. Since it is an S8, which was the top of their line in 2002 it might do better than other Audis, which have about the same reputation as modern Mercedes, except that Audi seems to be on its way "up" and Mercedes seems to be on its way "down" the reliability scales.
Lexus remains very solid.