Service history.. 500's get some serious hammering from their owners, so as long as the car has been maintained mechanically, you should be fine. Last thing you want is to pick up the bill for the previous owners neglect.
Don't forget to check the insulators that sit behind the rotor arm too - NLA from Mercedes but freely available as a Bosch part. They should be a relatively snug fit in the block to help keep some of the moisture out.