Yes, it is true: "Nothing is more expensive than a cheap Mercedes". You don't have much money in the car. How about pulling the motor and trans and finding a good used motor? You can't do the piston without pulling the motor out anyway. Used engines are available. I've even seen a couple on ebay. Might get you going for a couple thousand.

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Orland Park, IL
1985 300SD 215,000 miles