you are right, some of the 123 had a 5-spd in europe, some came over here as grey Mkt cars. very rare over here, but they are around.
Get in line, every one is looking for a 5-spd and the 617 flywheel. the needle in the hay stack.
If you have the money for the transmission and the shipping from germny, it`s doable. might be pricy though.
If you see the word Getrag, that is what they call them, or the co. that mfg them.
Do some searching, this has been discussed before.
Did you read the link in post# 10? the guy is converting a 190 5-spd.
this link will give you some transmission discriptions, and what to look for.
http://www.superturbodiesel.com/std/manual-transmission-information-t-421.html
Charlie