The manual transmission boxes that mercedes builds are better as the auto transmission boxes. The manual last very long. I had a W201 1985 190D with 600.000km and absolutely not a single problem.
The W123 240D had a strong manual transmission box as well; they can last a very long time, but some boxes are disabused or are out of luck and don't last that long...
(Brian) The later manual 5-speed transmission boxes seem to have more problems with their 5th speed as with the first and/or second...but some of the 5th speed problems can be repaired without the transmission box to be removed.
with a minor 600+k