Just a word of caution not all BM24s have dual Tuners so always ask and or check first.
Otherwise Yes if you go for a Bordmonitor with CD or Minidisk the CD or Minidisk will not work unless you mated with a BM54
Also remember the BM54 requires around £80 worth of BMW adapter cables as well!
Also whilst I am repeating stuff from other posts there is a thing called a "BM54 Back" which is a BM24 in disguise some people pay way to much money for those, I know because I sold a couple and was amazed at the bidding
Stew glad your MP3 Changer is sorted