I've been agonizing with a similiar problem for about two years and you'll find a wealth of information on the subject in the forum archives. I recommend that you spend the time to make several searches and you'll find that many have faced the same problem.
For information, the shop time for removing and reinstalling the transmission ranges from 4 to 6 hours. In the archives, you'll find a number of recommendations to use the Mercedes dealer for purchasing a rebuilt transmission. That route will certainly make sure that OEM parts are used, along with the latest improvements from Mercedes, but you have to be prepared to pay top dollar.

In the alternative, you might take a look at
www.mercedesdismantlers.com and www.peterschmid.com.
I sorta favored "Mercedes Dismantlers" and have made a couple of posts on the "Shop Forum" asking for feedback information from anyone who has used one of their rebuilt trannys, but never received a reply. Maybe you'd like to make a "post" asking the question.

I hope this helps.