I was in the same boat for the better part of 3 years I had been looking for a diesel gwagen, but could not locate one that was rust free. I ended up purchasing a CJ7 and doing a swap to a cummins 4bt. I am very pleased with this combination as this motor is much easier to work on than the 617, and is easily tuneable to double the output of a 617. The downside is that it is super noisy. The economy of my CJ with the 4bt I have surpasses the fuel economy I get in my W123. Personally I am glad I went this route, as opposed to spending more time trying to find a rust free gwagen that also has a turbo diesel. With that being said I found this example that looks pretty good (5th one from the top)and might be a fit for what you are looking for. PS...This is a great site to find Gwagens, its not updated that often but it is probably the only site on the net that has older Gwagens for sale....Good Luck!
http://www.clubgwagen.com/classifieds/vehicleads.php

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2005 SL65
1999 E300
1995 E300
1994 SL320
1988 560SL
1987 300TD
1982 300D
1955 300SL