e39 Navagation System

So I'm thinking about buying a 2000 528i with the factory nav system in it, however the Cd-Nav unit in the trunk was stolen, and the screen in the car does not come on, so my question is, if i buy a new Nav unit for the trunk will the screen come back on? and where can i buy a new or used nav unit, also would it be Mk II or Mk III? Thanks