USB Bluetooth adapters are very well supported. In fact, I would go so far as to say that you won't be able to find a USB Bluetooth adapter that isn't supported out of the box on any modern Linux distro (built-in and WiFi/BT combination cards are another story). This is largely due to the fact that there are only a handful of USB Bluetooth chipsets in circulation, so it is relatively easy to test and support them. If you are given the choice, adapters which use the Cambridge Silicon Radio (CSR) are the best supported under Linux, but again, any of them should work.
As for A2DP, current versions of BlueZ will have no problems supporting it, so just get a decent Bluetooth device and you will be set.