I'm not saying that an M5 will be a disappointment coming from a 530i/d but I feel that you should save the E39 event for when it'll be REALLY special and really amazing. You've already got access to an 530d - and as you said, you enjoy driving it but is that really what you want?
When you come to an M5 - do you want to think "Looks the same as my old car, just goes a bit faster" (ok prehaps a bit of an exaggeration but you get my point) or do you want to think "wow this is amazing I love this car, the looks, the performance, the interior".
When you come to an M5 - do you want to think "Looks the same as my old car, just goes a bit faster" (ok prehaps a bit of an exaggeration but you get my point) or do you want to think "wow this is amazing I love this car, the looks, the performance, the interior".