I am not sure if your car has ASR or whatever they might call it where it brakes the driveline.
But I do know that my little brother changes his brake pads every 10k miles in the front, but not the back. I think usually you go through two to three sets of front pads to one set of rear pads.
I havent had a car with ASR so I dont know. Ask one of the 500E guys. Doing brake pads is easy, you can do it yourself.
Also, the parking brake has parking break shoes inside the rotor disc, the brake pads are a separate system working on the same disc. If you took the wheel off youd know what I am talking about.
No Benzes in the stable at the moment, but itching for one again.
Former Mercedes in the Stable:
1983 300CD Turbo diesel 515k mi sold
1984 300CD Turbo Diesel 150 k mi sold
1982 300D Turbo Diesel 225 sold
1987 300D Turbo Diesel 255k mi sold
1988 300 CE AMG Hammer 15k mi sold
1986 300E Amg Hammer 88k mi sold
1992 500E 156k mi sold