I agree, if that is the only thing that bothers you, then grind it, paint it, undercoat it, or replace it. It sounds like you really like this car...black on black is a nice color combination, and, if it really only has 66K on the clock, then that is in your favor. The worst that can happen is you drive it until the 2 year warranty expires, then sell it for 8K.
You have been looking for 3 years, so how does the condition of this car compare to other W-124's?
Remember, if it doesn't work out then it wasn't meant to be.

Good Luck,

Dave