This car is a 1998 with only 39,000 miles, and appears to be in perfect condition. I'll be taking it, or a similar one, on a 2-day test drive in the next week or two, and will check the things you mentioned.
What I did notice so far are the strong engine startup and quiet running (the silence was actually a bit disorienting -- the car seemed too quiet - and I felt the engine more than I heard it). The transmission was silky smooth, and I was totally impressed with the power, handling, and true bank vault feel of this car. In reading the various threads about the W140's, someone said driving this car is like flying a jet, and that is indeed the sensation I got.
I was also pleased with the rapid acceleration, and how it handles hard acceleration with such grace and dignity. (I am sure many young hot-rodders have been shocked to have been effortlessly out-run by "gray haired gentlemen" in giant S500s!)
One thing I noticed was that the S500 I drove had about an inch of clearance between the top of the front tire and the bottom edge of the fender. On an identical S420, clearance was about 2 inches. Is the 5.0 engine that much heavier that the 4.2? I rather like this lowered appearance up front, which is actually lower in profile than my 260E.
I agree with you 150% that this is what a Benz is supposed to be. In fact, I think the 97-99 S Class is to Mercedes-Benz what the Great Pyramid is to ancient Egypt: the absolute pinnacle of their engineering expertise. I love it.