Are you planning on changing just what is in the transmission pan or flush all the system. If you are flushing just the system its a long process. If you are just changing what is in the pan, then it is easy. The pan holds only about four quarts. Since it will be mixed with another 8 quarts already in torque converter and in the rest of the transmission, I would only add MB transmission fluid. You don't want to mix ATF types. Will you also be changing the filter?

Like @mbfanatic above it depends which way you are wanting to go about it. From my experience it is good to leave some old tranny fluid in the system to mix with the new fluid u put in. But also while you are in there change the filter and the pan gasket. Also be very careful with the pan bolts alot if not all MB cars have aluminum bolts and are technically one time use bolts. I would replace with new ones but if you choose to use the old ones DO NOT over tighten or they will snap and you will have the stud of the bolt lodged I to the hole. A 2005 e500 I had required getting the temp of tranny up to a certain degree before adding the new fluid. I'm not sure how necessary that is but I did it anyway. 

Also a good time to check the electric connector that plugs into the top of the tranny as they developed leaks and can cause your car to go into limp mode. $15 pilot bushing will fix that problem even if it hasn't started yet. Then you know that everything is done for sure