As it happens, Spotify is currently offering 3 months subscription for 99p, so worth a try IMHO. With the iPhone Spotify App, you are also able to make songs/albums available offline, so no need for an internet connection. The audio is fed through the App, rather than through the native iPhone music player, but the iDrive can control the app to some degree (track fwd+back).
As a side note, I have an early 2009 vehicle with iDrive, so my Combox does not do Bluetooth A2DP (Advanced Audio Distribution Profile), I therefore have to use the Y cable to connect to the car.