Isn't it possible to to just fit a HID kit to the halogen projectors?
(I've done this before and it worked fine, was on a corsa though)
But my knowledge is a bit limited in terms of the 5er's, I didn't even know you could get OEM xenons for the e39.
@rabdos: what bulbs does your car take, as I have a H1 (I think) HID kit here at home.