Just buy a clean w140.
I owned a 97 s500 and currently own a 97 s600.
My s500 had the rear reclining heated seats among some other extra options. I really never used it. The rear shade, never used that either. I got it tinted.
As i said in my opening statement and have said this many times, alot of people with s600 really never have any problems with their v12, neither have I. Most of the problems are problems common on other w140 i.e window regulator, suspension etc... Of course, their might be additional labor charge, incase the mechanic wants to go around the v12 motor to reach an item.
I do like the look of the s600, in terms of the two tone leather (mine is black/gray), the wood steering wheel, and of course the V12 on the sides just for bragging rights
. I never push mine much, but i like how the v12 throws you back in your seat, if you step on it.
So, if had a choice between a really clean s500 and a clean s600. I will take the s500. Or you can combine both and get a grand edition s500, basically an s600 with a v8.