If you want the OEM sirius then you'll need the sirius receiver and the wiring. I'm pretty sure the wiring comes with the receiver. From the dealer this costs about $300. Many people have found the set up on ebay for about $150 and been quite happy with it.
If you want to just go with your starmate then you'll save money on a second subscription and all the hardware, you just won't be able to control everything from the stereo controls, or display things on the HU LCD. (bear in mind the HU doesn't auto-scroll info like many aftermarket units so you have to turn a knob to read everything). Anyway if you go this way you can just run the audio in through the ACI in the armrest. If you do this then you'll also want to get a ground loop isolator to avoid alternator whine.
