Rust resistance of the W116 depends on the year. One year (79?) there was a steel strike in Germany and inferior steel was imported for the cars.
Biggest drawback of the 116 in my view is the Evil Servo.
W116's also had (have?) a problem with the ears breaking off the input shaft of the transmission due to the incredible torque of the 'new' turbo engine.
There was also a recall on 116's for the crankshaft pulley.

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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