You can do the Sirius with a dash-mounted antenna (it's about the size of a quarter. It's unobtrusive and does not have to be permanently installed, but you'll get much better reception with an exterior antenna. Also, the Sirius music channels ARE commercial-free. I think you can do an online 2-day trial to check it out first.
Otherwise, the iPod (specifically) is the way to go. I have an iPod specifically for my Challenger (it has only '60s & '70s music) in the Chally at all time. You can get an 8GB iPod (about 2,000 songs - more space than you'll need) for about $150.
Mike