I have done one for myself. 2003 4.4i X5 with MKIV and 16:9

If you have the proper tools, you can do it for yourself. It took me about 2 1/2 nights to get everything stripped and installed. The hardest thing for me was to run the retro cable from the back to the dash and getting the dash modified with drills and files - it is worth it!!!

If you are planning on getting the major parts from eBay, your-miles-may-vary on the price. About the year ago, I bought the retro cable and misc parts from Crevier BMW and that cost me around $500-$600(if I remember correctly) The major components, I was able to get them from eBay.

Oh, my car already had SA55(High-OBC?) installed already and the GPS antena was already installed in the spoiler even though it was not connected to anything! If you need to upgrade with SA55, that would be extra and coding is required by the dealer for the instrument panel to work.

I took some pictures of my retro so if you want I can send you some of the pics.

Good luck~