If the car's starter jumped to life when you did the solenoid trick, i would say its a starter solenoid for sure. Not sure if Mercedes starters have removable solenoids or not. If so, this will be an easy fix.

If the starter/solenoid are all one unit and must be replaced as such then i'd see if you have a shop locally that can rebuild the starter for you. Diesel starters are quite powerful as they have to push the crank and rotating mass as well as fight extremely high compression. Which means they'll probably cost more to replace/rebuild (rebuild being the cheaper of the two)

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93 300 D 2.5 turbo