Hi there,
It could be that the previous owner installed pads with a high metallic content. The original Mercedes pads have a high organic content and work very well, but "shed" a lot, leaving the wheel rims soon dirty. You might try some new pads from MB to see if the "grinding" disappears.
My '82 has a noise when going very slow and if I step on the brakes I hear a noise, but it is the exhaust pipe hitting the body due to the torque of the engine... probably needs new motor mounts.
Another thing to check is the front wheel bearings. Make sure they are in good shape and adjusted properly.
Congrats on acquiring a great car!
