Does the Zumo 550 have a built in XM radio? Other than the antenna and a subscription is there anything else that I would need? Also I have a bluetooth headset so I should be able to listn to the XM through my headset right?

The Zumo 550 has the XM tuner built in, but the axtual receiver and XM radio ID are in the Garmin XM antenna. 

I use the Bluetooth for my cell phone in my car with the 12v powered car adapter that comes with the Zumo.  It then becomes a hands free speakerphone. You can dial and answer the phone from the Zumo screen.

The bluetooth does not work with the XM for music.  On my Vision, I used the Aux input to get my XM through the speakers on the audio system.  I still use the bluetooth for my phone, but also pair my bluetooth earpiece with the Zumo to use the phone while riding.  Also, if a headset is paired with the Zumo, the turn by turn direction are through the bluetooth headset, not the audio system.