I'd go with the torquier and newer 520d. Check for the usual like suspension, if it was flooded, if all buttons work, if the engine emits smoke when you open the oil cap, if it doesn't overheat when driven hard (it never should), if the AC can make you shiver and numb, etc. the engine should sound good. One way to do that, is test a friend or someone's e60 diesel and compare with what you are intending to get.
You probably can change to non-runflat tires but does it have a provision for a reserve tire,even just a donut (superthin spare)?