Thinking of purchasing a 1995 S500 sedan with 52K verified original miles on it. What are the pitfalls of this model? Is it reliable?

Wiring harness, evap () and tranny are always discussed. Maybe with the low mileage you're in good tranny shape.
Those are generic W-140 issues. Others will chime in on other issues with that chassis and with the S500 specific things...

__________________
Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John

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.