When your brake pads are worn down, to the point of needing replacement, you should see a warning light on the dash. It could be, however, that the previous owner, at one time or another, changed the front brake pads and the sensors were not reinstalled.

Since you say you're getting a noise in the right front wheel when you "hard brake", there's obviously something going on and I suggest that for starters you remove the right front wheel and take a peek at your brake pads. I usually do a quick inspection anytime I take the car in for wheel rotation and balance.

In the meantime, I'd try to avoid hard braking because if the pads are completely worn down to the metal, you could be trashing your right front rotor.

I've found that anti-squeak lubricant will usually cure squeaking brakes. Also, I never use anything other than Mercedes brake pads because I once had a horrible experience when I made the mistake of using aftermarket pads. It's my understanding that Mercedes uses what they call a "soft pad" to minimize noise and anything else can cause excessive heat and warped rotors. Warped rotors is precisely what I experienced.

If you're looking for the cost, you can't make a reasonable estimate without a full inspection, front and rear. There's always the question of whether or not rotors need to be replaced and if so, you'll be looking at a higher cost. (As a minimum, most brake shops will "turn" the rotors on a lathe and bring them back to "smooth and true" condition unless you tell them otherwise.) You'll also want to inspect your calipers and hope that there's nary a sign of fluid leakage.

I hope this helps.
William