Well, I'd say as far as fuel economy with the exception of the 300SD and non US 280SE's, all other 116's will be gas hogs no matter what.
We in the US got short changed with the 280SE because our cars only had 137 to 140 hp and never got more than 18-20 mpg, whereas in Europe an unrestricted 280SE had 185hp DIN and was good for 25mpg at 100-110 kph. MB's solution.... the 300SD... same performance as a US 280SE (actually better...) and close to 30 mpg, so for the States the best compromise between performance and fuel efficiency is no doubt the 300SD.
As for the the 450, again US cars got hurt big time because of emissions, the 1980 M117 is actually pretty good, on the highway at steady speeds I have seen 25-26mpg with the a/c off doing 60mph. Torque on the O2 equipped 450's (only 160hp...) is very decent too.
Early D-Jet US M117's deliver terrible gas mileage, so so performance and 75 cars have the dreaded cats on the exhaust manifold responsible for virtually frying everything under the hood.... no wonder MB went to K-Jet and an under the floor cat in 76.
One may ask... what about the 350's and 280S?
350: Great cars, the availability of a 4 speed manual makes them super desirable in my opinion, however they are marginally better than 280SE's in the performance department and not exactly frugal on fuel.
280S: Euro versions are quite good but they don't offer a lot of performance and gas mileage is pretty close to a 350. US versions if fully original with all the smog equipment will be as bad as a 450 on fuel and with only 120hp they can't do much...
So what I am trying to say is that if one is concerned with gas mileage and still wants decent power in the US the 300SD is the best option. For the rest of the world the 280SE is probably the best compromise, hence it being the best selling version of all 116's.