I think you need to put valve guides and seals in the 500 euro and enjoy it. I am saying this because my 500SEL US version actually has a euro 500 engine that has just as much power as any 560 I have driven and gets 20 mpg on road trips and even better if I can drive it 80 mph or better. From what I have read on this site, 17 to 18 mpg is about tops for the 560. Another thing, the 84 and 85 (maybe others) use regular gas.
Why do you think you need a rebuild? The 117 engine is bullet proof except for the timing chain breaking and the valve guides wearing out.

Paul

__________________
84 500 SEL (307,xxx miles)