They are dificult to get at. I work as a shop manager and had a mechanic pull out the blower motor on my 300e , it took him a few hors to complete the job. You access it thru the engine compartment and have to remove the wiper assembly.
The motor cost about $300.00 and is available only thru mercedes, you can also option the cheap-way (as I did) and buy one from your local junk yard, It wont be dificult if you live in a big city, for about $40, just make shure it is tested and operational before you buy it.
As far as I know there is no brushes replacing on the motor, I do not know if there are internal parts available for this motor.