My thoughts are fix everything else first, the easy stuff before you tackle the bigger jobs like a diff swap. Things like injectors and fuel quality can have a major influence on mpg, timing (chain stretch and start of injection), low rolling resistance tires/narrow tread on the road (there is a big difference 185 vs 215 width). I knew many owners who absolutely flogged their S classes back in the day at 90mph constantly before there was any concern over speeding, fuel prices or traffic flow. Those cars lived long lives and the owners never complained about the fuel economy going down. I seem to remember 30mpg (USA) being very easy at any speed so long as you stayed out on the highway.
My point is they were built to run hard-they can take anything traffic today wants to dish out. 3500+ rpms not a big problem. It is up to you to decide if can afford to fuel it, and keep it running in top shape.
Running a 2.47 diff would certainly require everything to be in good shape like pump calibration and timing, I have driven some with stock diffs and they could not make a left turn without getting run over! Each car is different...