Hello,
I do not claim to be an expert here, but from what I have read 750 miles to a quart of oil is not all that bad, yes I agree that that's to much oil!, but apparently MB says 500 to the quart is acceptable, (maybe if your an Arab).My 91CE with 140,000 can choke down a quart in 1000 miles if I drive the "hell" out of it. It runs great, so I just add oil. Probably your valve guides are worn.
As far as what a rebuild would cost, I have no clue (if you have to ask, you can't afford it) any guess's out there, it would be nice to know what our options are, as we all will be there some day if we keep our W 124'S. My guess would be $ 8000 for a complete rebuild, what's your's
Scotty