Re your region, this SF Bay guy is asking (actually demanding) $26,500 + $1500 delivery for a "super-rare" 1985 300D
https://sfbay.craigslist.org/sfc/cto/6151210722.html
It does have new belts and a battery and only 96,500 miles, so certainly worth more than asking price. Sounds like a guy who will rant and spit if you offer only $20,000. If you see it up for a year, that might bracket your expected selling range, or watch what it goes for on ebay (which he plans).
Personally, I consider my 1984 & 85 an ender's game. Some critical parts are extremely expensive or "not avail". The engines can't be affordably rebuilt, though I have a spare one in-work. But, w/ hot-rodder mentality one might re-purpose the cars. If I ran out of engines, I might consider a spare Mopar small-block and tranny I have lying around and the IRS would let it perform well. I am not wedded to diesels (like most here). Having 20 gal of gasoline behind the rear seat is not as comforting as diesel, though many U.S. pickups did that. If the M-B dash proves too much, one might retrofit a complete dash w/ climate system from a newer junkyard car, and same w/ seats.