The first things that I would check in regards to the stalling issue are the distributor caps, distributor rotors, spark plug wires, and spark plugs. The early M119 engines (prior to the ones that use the coil packs) are sensitive to distributor caps. Also make sure that the engine has the correct spark plugs installed. None of the spark plugs listed in the owner's manual for my '98 S420 are made anymore, and if you look in the spark plug books at the auto stores they recommend a different number with a resistor. So there is a chance that someone may have installed the wrong spark plugs. For example, the non-resistor spark plugs listed on the Bosch and Beru web sites for my S420 have a lower heat range number than the recommended plugs in the manual.

The door panels on the W140 are more difficult to remove than the W126 panels. I believe there is a DIY article on how to remove them somwhere on this site. There are airbags in the front doors, and even though the bag stays on the door when removing the panel it's something you should be aware of. But since you've owned two previous MB vehicles it's probably within your skill range. It's simply a little more tedious and time consuming than removing W126 panels.