Aftermarket radio questions for 2004 M3 Convertible

I just got a 2004 M3 Convertible (LOVE IT!) but want a different
radio. (This one doesn't play MP3's or have satellite. I know I can
get a different BMW radio but want to know what my options are.

Here's my questions:

1. If I put an aftermarket radio in, will it "mess up" the BMW Assist
or any other features on the car?
2. Can the current controls on the steering wheel work the aftermarket
radio? Best Buy says that they have put adapters in that allow the
steering wheel controls to do so BUT crutchfield.com says there is no
such adapter for the M3 or the 3 Series (and I couldn't find one for
post-2003 cars through Google).
3. Any other info you can provide for this process would be
appreciated.

Thanks.