If the soundsystem is that important for you, you'd better go for a 640i or 650i. You can order it with a truly brilliant Bang & Olufsen system. Make sure you have BMW Professional navigation, BMW Online, Bluetooth and iPod-integration. It's state of the art and almost sounds like a concert in the Royal Albert Hall.
On the other hand, you can have some fun with the M3 too.
Can't remember it too well, but I suppose that car had a radio. I didn't use it anyway.