I have not done a more recent MB tranny. I did a 01 E320 but that has fill tube. My BMW is much older but doesn't have any fill tube at all and this is what you will be dealing with.
I think it is about 6 quarts or so if you get the torque converter drained. Always buy more than you need.
Tranny fluid... MB tranny fluid or what Amsoil specified for your car. I use Amsoil fluid for MB tranny.
As for filling the tranny, they most likely have two plugs... one to drain and one to fill. The one to fill, you have to 'inject' it into the transmission. Then you need to start up the car and let the fluid flow... stop engine and fill up more until fluid splash back out.
The tricky part of the filling is that the tranny must be level with the car and engine is running. It is much easier to do this at a shop than in your own garage.
Is the car still under warranty? If not the car, tranny? Call up your dealer and see if they will service it under warranty for you. Powertrain warranty is a bit more coverage than the rest of your car I believe.