Unknown about the company, but basically find out if they specialize in MB trans, are they Dyno tested, are original parts use, are there any relined bands used, what is the warranty parts and labor, price, does it include the torque converter?
Those are the kind of questions you can ask the supplier.
Mercedes-Benz Master Guild - 2 years
Shop Foreman - 20 + years - dealer.
Part owner - MBZ Transmission remanufacture