Stereo upgrade questions

[Meinhold]

Hi everyone, I'm new to this forum. I just bought a 03 x5 3.0 sport package. I have had and have many BMWs. I currently own a 2000 528i and 2001 740i. I really love my cars so I found this x5 for a great price, so thought what the hell. I have had it now two weeks and it is really growing on me. The question that have is; I want to upgrade the stereo but I can't find much clear info. I have currently none nav. with dsp and a sub woofer in the back. I have seen the complet units on eBay and I put a similar one in my Excusion and it it's that bad, I kind of like it. So here are some of the questions I have.

1. Will I be able to use the bmw amp.
2. Can I keep using the on board computer, like gas mileage, time, date and ect.
3. The aftermarket units say the steering wheel button will, but will they.

Any other advice would be great, thanks forany help.

Tom