help! I'm totally lost ...

[haumana]

I'm a newbie X5 owner. I want to change the stock stereo, but am totally lost about it. Here are the details of my ride...

2004 4.4 (mfg. 9/2003)
No Nav
No DSP
No BT
No BMW Assist

My end game is to have either a single DIN or double DIN head unit that has a USB port in front. I do not need GPS or NAV or DVD; Bluetooth would be nice, but not high on the list. I live on an island and at best, can only drive around in a circle. No place I go is far enough for me to provide video entertainment for my passengers. I prefer to listen to my MP3's via USB over Disc, iPod, or Bluetooth.

Do I need to replace all the speakers if I just change the head unit?

or will the power from the HU and a wiring harness take care of that?

Will my steering wheel controls still work with a new HU, or do I need an additional adapter?
If I can do a simple HU swap, is there a minimum wattage I should look for?
Does anyone have a setup that they can post pictures of?

I'm guessing that even though my steering wheel has the phone buttons, that my X5 was mfg too early ... so there probably won't be any wiring in place, right?

Any help is appreciated!!

Thanks in advance!!