Check out the Shop Swap Classifieds. There is a 190E with an M104 and 5 Speed for sale. It was built by someone at Patomac, a large salvage yard. If I was around some place like that, I'd find out what would fit what.
I want to research this, I don't know how helpful the fellow at Patomac will be. It seems that the 5 Speed that's hooked to the 2.6 in a 201 car is the same transmission. If this is the case, the transmission has some availability. If a person could get the pedal mechanism with the master cylinder and the other parts, you could get enough stuff to build an M104 with a 5 Speed out of an automatic 300E. There are lots of them around here with good bodies, but maybe tired engines or transmissions.
I would like to do this instead of changing my original 124 5 Speed car.
Anyway check out the classifieds on this site, and have a great day.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in