I don't believe they offered a belly pan for the 124 Diesels. ke6dcj had one on his 360TE wagon while he drove it on the W Coast, you might ask him if it was custom built.
Does you car have the bottom covers on it? They may help protect to some degree but if your running a sport suspension (lowered chassis) or just plain have a lot of pot holes or ruts where you drive then a steel belly pan might be a good idea and a good shop should be able to fab one out of aluminum diamond plate

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!