Mercedes Benz Classic magazine has been running a series of articles on 'Modern Classics'. Cars that don't fit the criteria as classics yet but are destined to do so one day. In their Q4-08 issue ("Weighty Choice", pgs 46-51) they covered the W140. Areas mentioned for special attention were power steering pumps, AC evaporators and valve cover gaskets. On the V8's and V12's cable harnesses and CATs were also mentioned. As a rule cost of ownership goes up with engine displacement.

There's the owners club for S Class:

http://www.s-klasse-club.de/gb/index.html

Here is the magazine's site in case you want to try to get a copy of the article:

http://www.mercedes-benz-classic.com

Good luck.