OM, we seem to have common ground here: I had an Eldorado with the infamous 4100 engine that kept me pretty busy for about 9 years.
For the last 17 years I have had a pristine '91 300SE, and it has just about 180k miles, as yours does. I have had the usual water pump, alternator, idler pulleys, belt tensioner issues; I attribute much of this to the high revs the engine turns in the S-class car.
I also replaced the head gasket several years ago. The reason the head gasket fails is simple material properties; you have a very long aluminum head sitting on an iron block. The difference in expansion rates causes the head gasket to drag over the deck surface every time the engine heats and cools.
Mine manifested itself as a water leak near the right rear. The valve stem seals were lousy from the start, with the engine always burning oil on the highway. I did the R&R myself (pretty easy to do) and had the head freshened at a local shop, including valve job, guides, seals, and planing, for about $300 or less, I think. Since then, no oil usage, and it runs great.
There are other little issues on the M103, like the front cover seal that always oozes, and sagging motor mounts. But the head gasket issue seems to be the only major problem; and fixing that every 10 years or so is not in my mind an indication of a non-durable design.

Would I take this car on a long trip - no, not since it stranded us last year with a busted belt tensioner right out of the blue. But, do I trust it to get my wife safely to work every day for the next 10 years - absolutely.

DG