It sounds like this guy is throwing parts at it and I would find it hard to swallow a $1200 part in that scenario. I would think about taking it to an Independent that specializes in Mercedes. They probably work on M103s all the time.
Where are you located?
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino