I have a 1992 400e. I get CONSTANTLY 19-20 MPG. This is in combination hwy/city driving. At each fill up, I log my mileage and fuel used.
Hwy driving is mostly around 75-85 MPH with WOT runs up to 115+ whenever possible.
City driving includes "jack rabbit" starts at all opportunities.
As for working on the M119, it is not that difficult. I also have a 1988 300e. The 400e is a bit more difficult to work on, but not by much. The increase in difficulty is maily due to the tighter working conditions.
Pros: Fast, solid, and good looking.
Cons: ..... hmmm..... it's not a 500e