The timing chain will stretch over a period of time and can break. There are guide rails for the chain mounted in the front of the cylinder head. These can break as they get brittle and get caught between the chain and the sprocket causing the engine to jump timing. Either of these can cause extensive damage. It's worth the cost to replace if you have mileage 100,000+. As far as the brakes, let me give a short explanation. The calipers squeeze the brake pads. the pads are squeezed against the rotors. It's most likely you are wanting to know if you'll need pads and rotors. You'll need rotors if they are worn below the minimum thickness specs or if they are warped. Usually Mercedes rotors don't leave enough material after some use to machine. Caliper problems generally occur as a result of leaking or binding, but don't give a whole lot of trouble. Hope this attempt to explain will help!
Mercedes Star Technician,
Mercedes Master Guild Technician, ASE Master Tech. 18 years exp.