A search on the W140 chassis will return several of the same type of resonses you are seeking.
From what I can tell, this drivetrain is very solid. Aside from the OM603 3.5L diesel, the engines in the 140 are fine. Common problems with the earlier 140 body cars are with the AC Evaporator (big $$$), closing-assist pumps (expensive, but not unbearable), HVAC control units (expensive, not terrible), and worn suspension parts (sometimes expensive, others not so bad).
A comprehensive extended warranty wouldn't be a bad thing to have on an early 140. Especially important would be a very thorough pre-purchase inspection by someone familiar with these cars and their related issues. Finding a trustworthy, knowledgeable mechanic may be an issue as well.
The common problems aside, the 140 is one heck of a car. Maintaince is on the high side, but if you can swing it, it will be a rewarding experience.
Sold: 1987 MB 300SDL, 320k miles, Astral Silver, Blue Leather, all books and records from day one.
Totalled: 1998 VW Jetta TDI, 488k miles, Classic Green/Gray Leather, loaded, E-Codes, "Slightly" modded, "Slightly" faster than stock, arrived 11/07/1997.
MBCA Member, Chicago Region