My .02cents worth....
Skip V12 model.... S500 offers the BEST compromise of performance and gas mileage of ALL W140..... it really depends on what you are looking for...
My W140 was a daily driver for my Father since March 1992, and up til 1995, when he decided to stay in Asia longer than in Canada(business), he pass on the car to me, and since then, it has been my daily driver 'til recently after I aquired my TL Type S(my brother is driving it now)....
'97 models should have all the bugs worked out, and would be a very rock-solid car. Skip 1996 if possible, first year with 5-speed electronic tranny, and heard there were some glitches....
I have listed couple lists regarding changes done to North America W140s through out its life cycle(might miss things here and there... :p).... hope these would help....
http://www.peachparts.com/shopforum/showthread.php?threadid=7592
http://www.peachparts.com/shopforum/showthread.php?threadid=8335&highlight=SClass
Hm... there should be a much detail list, but I can't find it some how...
Anyways, keep searching, it's hiding on this site some where... heh
Andy Kuo