Are you buying it privately or through an independant dealer?
None of the problems you mentioned are serious, and could be fixed fairly easily by you if you have the time.
As I'm sure you're aware, be wary of a Mercedes with no history. Especially a cheap one..
Generally you'd expect a 420 S-class Long wheelbase to be well maintained and looked after, including at the least some service history, so make sure you convince yourself there's a genuine reason for lack of history (and there's plenty of them).
If you wanna be really picky and sure about the car, I would check things like:
Does the AC still blow cold, do the tyres match, are first aid kit & triangle in place? Might give some indication as to its past, and its recent maintenance/lack of. And of course mileage could easily read 100k but be more like 250k.
But I'm guessing you know all this and more already, so with all that said, a cheap W126 should still be solid and run for many more miles if you're prepared to sacrifice a little money and/or time.
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black