what is the oil leak all about? iis it an easy fix with the 545i?

That depends entirely on where the leak is, could be minor, could be major but you failed to mention that...where is the oil leaking from?

Tom just to correct you the engine this vehicle has is a N62 and what you are saying is for a M54 engine found in the 525i for the e60. The oil leaks for the N62 is most likely coming from the valve cover gasket and upper timing case cover gaskets. These are a very pricy thing to repair a lot of labor involved. Another area it could be leaking is the transmission sealing sleeve and pan. This is also very pricy repair. I would get a lot more info on this leak before purchasing the vehicle and see it for your self.