You did not say what model MB you are working on so I'll have to assume that it is the '87 300D.
That being the case, with all you said, the BIG deal here is the evaporator since it is so difficult to change. Everything else just doesn't matter much since you can replace any other component relatively easily, save maybe for the compressor.
Since I HIGHLY believe in keeping a 124 car on R12, and the car you are putting on the road is R134, this means you would have to remove the compressor to flush it and change the oil for use woth R12 so that is the same work either way.
This is what I would do if it were me. I would do everything I could to leak test the evaporator. You should be able to do this at the firewall next to the master cylinder since that's where the expansion valve is located.
Once I was happy with the evaporator, I would flush it THOROUGHLY and then flush the lines thoroughly. Then replace the filter drier and condensor after, you guessed it, a good flushing. Then take either compressor, whichever one you feel comfortable with and flush it by pouring in mineral oil and turning it by hand to get out as much as possible. Then put in the correct amount of mineral oil, use new o-rings and nylog everywhere, pump it down and charge it.
I would NOT change one of these cars to 134. All of the work you will have in it makes R12 a small price to pay to reap the benefit of your labor.