the Land Rover nav computers that are compatible with the M5 are not MKIV but MKIII, which is the same as 01, 02 M5. You can not change the firmware with current hacking tools, but with a hacked software cd you can change the logo to ===BMW=== But will still have the Land rover User interface wich is old 2001 software and sucks. It was never updated even in the 04 Range Rover. And Kris Linquist firmware load will not load on it either.
Do a Google search for BMW MKIV navigation. There are places out there to get them. I would recommend buying from experts (forum members, bimmernav, ect) instead of off ebay or something