I have a '93 400E with 87,000 miles -- it's essentially the same model as yours. My mechanic said I have some seepage from the left bank of cylinders, but I haven't seen any oil on the ground nor does my oil level seem to drop between changes. I have had some seepage from somewhere in the power steering system, had a few hoses replaced, and still have to top off the PS reservoir every 6 months or so.
While mine was still under an extended warranty, the dealer overhauled the fuel pump(s), replaced the water pump, and replaced the starter. Off the warranty, I had my evaporator replaced (probably the biggest Achille's heel of the W124 series) to the tune of $1800.
I too have asked the kinds of questions you are asking, such as why such a well-engineered vehicle should need repairs like I've described. I don't have any good answers, but, knock on wood, I hope for a long spell of no major repairs for a while.
FYI, I did switch to Mobil 1 synthetic at 85,000 miles, having read such good things about it on this and other lists. I'm hoping to preserve my valve train as long as possible, and the same with the timing chain. I haven't had any problems with leaks even after the change to Mobil 1, despite some of the horror stories you hear when higher-mileage, older engines are swithed to synthetic.