First thing to do is swap the front tires side to side. I have seen this problem several times where pulling is caused by the tires and reversing the direction they turn fixes the problem. I have never seen this happen because of rear tires but I guess it could happen also.
Also, the amount you need to turn the steering wheel to correct for the pulling bothers me also. On a 126, turning the wheel from say 1 o'clock to 2 o'clock is a lot. The wheel should be straight at all times on a straight road. There must be an alignment problem somewhere. There is an alignment hole on the steering box that needs to be set when doing an alignment. I'm sorry, I can't remember much more than that it is there somewhere and should be used to make the steering wheel straight while going straight. Most alignment shops just tie the wheel straight and align everything else to that. Mercedes cars need the steering box locked in the straight position and then align everything to it. I hope that makes sense.
Look for the vibration after the other problems are fixed. These cars are very sensitive to tire balance but out of balance tires won't cause pulling.

Just my thoughts.

Paul

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84 500 SEL (307,xxx miles)