As you know the 123 five speeds are very rare in the US. My Euro 240D is a four speed. I would really like to have a five speed 240D with the right rear ratio set.
Now that this thread has migrated toward mileage on these transmissions and clutches:
'88 300E Five Speed 251,000 miles with original clutch and transmission has never been apart. Everything still in perfect shape.
'84 Euro 240D 533,000 miles, third clutch but transmission has never been apart.
I find the comments about the MB manual being "unrefined" kind of amusing. It is really no different than any other decently built constant mesh manual transmission. What could be "unrefined?" I always seem to hear such comments from someone who actually prefers automatic transmissions.
I have shifted everything from Volkswagen bugs, to '48 Chevys, to an Army Deuce and a half. If you are competent with a manual you really never know the difference.
Have a great day,