You're not going to find an MB turbo 616. MB never made them. The only place you can find them is in India where Force Motors (Tempo?) got the license from MB to build a turbo version. They are still building them. There was a recent thread about them.
Lots of people have put an aftermarket turbo on the 616, particularly in Europe where the 616 was a very common motor in trucks and motorhomes. I've talked to a man in the UK who did it to his motorhome and was very happy. I have no idea what kind of boost he was pushing and how long the engine lasted. Motorhomes tend not to accumulate lots of miles so perhaps it's a moot point in that application. I've posted a link in the past to the UK company that specialized in turboing the 616.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13