I have an alpine head unit with Sirius I pulled out of my last truck. I'll cut you a hell of a deal if you are interested in something lightly used. All you will need is an antenna and the right bracket for your dash. I went with a nav. system in mine.
if you want to keep the factory radio, you can check peripheral electronics for a aftermarket satelite radio unit interface to integrate it with the factory head unit.
During a visit to the parts department at my local dealer I learned that the dealer was currently running a promo they run ocassionally with Sirius for a free factory unit, free installation and free service for 9 mos. They gave me deal even though I had purchased my vehicle sometime before.
I have also seen free units with installation if you sign up with 12-18 mos in advance at some audio shops. I would suggest checking your local area.