I've heard that the M112 also suffers from a higher incidence of MAS failures, but all new cars are suffering this problem due to the new MAS design.
I know a couple people with higher miles 98 E320 cars with the 3.2 version, and it seems that ignoring the FSS is the key to making the M112 last. They use synthetic oil, and change it much more often than the FSS calls for. I would NOT buy a 98+ MB used car that was "serviced by the book." Much more frequent oil/filter changes are called for.
The 722.6 seems like "luck of the draw" and problems seem to show up early. Both our current 98 models have the five-speed and they have been perfect. I had another 98 C230 that was not so lucky, and the transmission failed completely by 6000 kilometers. My friend Kevin has a 1998 C280 that also had tranny failure. My Dad's 1998 C230 has 70K-kilometers, and no tranny problems whatsoever.
Don't be fooled into thinking this is unique to MB. I know a few Acura owners, and the new TL five-speed auto is HIGHLY failure prone. I rode in a Lexus IS300 a month ago that had a slipping five-speed auto, and this problem seems common over at Lexus.
Were I shopping for a 97/98, I would probably pick the 98, and go for the V-6. As to 722.6 problems, well, I love having five speeds, and will endure teething problems. Changing marques seems to make no difference with transmissions.
1998 C230 "Black Betty"