I don't think your mechanic's argument has any validity. The W124 was a well made car from the start until the end of production in 1995. I've sat and driven in a late W124 and they feel as every bit as solid as a early W124. I don't think the decline in engineering and material quality came so early. It was more of a gradual transition starting in the late 1990s.

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1989 300CE