I'm living in the city (L.A.) and constantly having to parallel park. Can the park distance control be retro-fitted to a 2000 m.y. E39 that did not originally have it?
I think it can. Seems like I remember seeing that in the BMW options catalog at the local BMW dealer. Call your local/favorite dealer and verify.
Definitely inquire of the parts department at your local center. For some vehicles you have two options: the OE-style system or a rear-only kit integrated with the license plate holder. The latter is significantly less expensive and applicable to more models.
I saw a DIY link on this subject. Looks like about $400 in parts. There is an actual kit from BMW. Apparently a speaker that indicates via audio, not a visual LED display (like I had on a rented Mercedes 180 in Germany). I will be looking into this further.