I have watched the classified prices of 500E's for awhile now. (even before buying mine) They seem to be pretty close to a "bottom" level for the early cars, the E500's still having a bit further to slide. Beyond that, it is hard to say. 500E's are certainly not as problematic as the previous factory hot-rods. (6.3 and 6.9 owners know huge up-keep bills are a fact of life) The E55 has created a flood of used 500E's on the market, before then you never saw them used.
Call the dealership or independent where you plan to have your E500/500E serviced. (and other candidates in your area) Service and repair costs can vary greatly between geographic location and individual establishment. Costs should be no worse than other M119 W124's. Service items and itervals are pretty much the same as other W124's.
Hope this helps...Lee