Use only MB fluid.

You can't drain the torque convertor, but for the rest you can follow my DIY guide based on my 722.6 transmission in my W210

http://mbclub.co.uk/forums/showthread.php?t=23969

As for the torque convertor, you can replace whats in the gearbox and sump, re-attach the pan, run the engine for a few miles, and then drain some more off - if you do this twice you'll get most of the old fluid out.

Or you can disconnect the ATF cooler pipes, run the engine, and flush the system that way by pouring new fluid in as the old stuff comes out. Please note that I've only heard of this being done on other cars, and not an MB.