My first recommendation would be: buy the manuals! It is the BEST investment you can do if you plan to keep your car for a while.
If you can live without your car for 1-2 weeks (depending on our pace), you can do it! and save BIG $$$$$. However, it is physically a very HARD job.
Before starting I went to a MB independant and "evacuated" the freon. Then, I replaced the evaporator myself along with the actuators, the reciever drier and the expansion valve. After that, I took it back to a MB independant to change over to r134. The r134 does NOT cool down as good as the R12 specially at idle but, it does the job (I live in Florida). Keep in mind that the AC system in the early 300e was not designed for r134 plus it is NOT the best design in the world to begin with.
I used an after market copper evaporator. I don't remember the brand. No problems since the replacement.
As far as the job itself, you are not only going to remove the dash but, you have to remove the AC blower assembly, which is done from the engine side. Some of these instructions are NOT in the manuals.
If you are NOT mechanically inclined, pay your mechanic to replace it for you and do the change over r134 at the same time.
You will be glad you did!