I had no idea that BMW had a Montana based Navigator. Looks to be based off the 600 and should be a good unit. Map updates should be free, so you should be good to go.
The powered AMPS style powered mount that you can buy from Amazon/ebay/etc have a little security torx screw that does lock the unit into place to some degree. Just ensures that button isn’t able to be pressed and thus ejects the unit.