Your buddies at v12uberalles.com don't have an answer for you? You'll get better responses in the Tech Help forum than here since W140s are a minority here. The 94 or 95 headlight upgrade is something to consider. Fog lights moved to the slits below the main beams and high beams moved inboard to where the fog lights used to be. Only minor rewiring vs an HID conversion. If you're set on HIDs, see if there's a deal on OE units at car-part.com . The folks in the Tech Forum can tell you what you need besides the headlight units (ballasts, igniters, connectors, etc.).

There's a fellow I don't see in DD anymore who had a late W140 with HID bulbs in halogen housings. They worked very nicely, IMO.

Sixto
87 300D