Update us as you make progress, if you can. I've been putting off tearing into my non working sunroof for a while. I got some high estimates in the $300-$500 range--not including electric motor. I found the CD service manual to be pretty useless, but I ordered the new Bentley owner's Bible which is supposed to include the well-kept secrets that surround most W124 repairs.
I'll let you know if it has any promising tips regarding the sunroof.