Ditto to Michael's response as well, I think the required repairs you mention in your first posting are not out of the normal, after all the car is 9 years old.
Concerning your question concerning the rear self-leveling suspension, I would say you only must perform the recommended fluid flush and your system should stay healthy. You may need to replace the spheres every 100k or so, give or take 20k depending on your driving habits and general road conditions.
I'm not sure what is going on with your rear drive-line as it does seem as if the PO was chasing down some sort of nagging problem there? Not common to the 500E I believe.
Other than that, it appears as if the PO has chased all issues out of the building, W124's do not seem to be "Lemons" the way other cars appear to be. With all the above mention repairs the PO has had performed I would guess that the PO was 1. very maticulous with all issues that appeared, 2. was very hard in his driving habits which wore the car components early, or 3. happened to own a problematic 500E? You general impression upon inspection may or maynot be able to give you the true picture.
Is this car being sold by a Dealer or the PO?
Where are you located, there are many owners in So CA and several in the Bay Area?