I agree with getting the "factory" (rebuilding centre) rebuilt unit if you can.
I don't know if the 1997- 722.6 five speed auto will bolt in, but I suspect not. The 722.6 five speed is electronic and probably has a different interface with the car. But, the conversion might be possible. Any techs want to wade in on this point?
BTW, 223,000 miles is pretty good life for an auto-box. You got your money's worth so far...
1998 C230 "Black Betty"