This topic has been discussed before. To get the total history run a search. The nutshell version is as follows: Go with an aftermarket extended warranty. The first "big" thing that goes wrong and the warranty will of paid for itself and a night on the town. I have had excellent personal experience with Warrantech. I'm sure there are others out there though. As far as what to look for at the end of a warranty....you can't exactly go to your dealer and say "check out everything on the car its almost out of warranty". I believe the popular circumvention is "I'm going on a long distance trip and need to make sure the car is okay/safe/etc." Have the evaporator core checked out on that W140 as well as the head gasket on the M104 motor. Talk about a nightmare of a service bill...
Hope this helps,