fyi: You will be flamed.
There are many FAQ's avvailable that cover all of this.
http://www.digitalww.com/DWW-6500.htm) for $650
or you can get a motorized flip-out single din 7" unit like the following for $450 at the mp3car store or one just as good for about $250 from eBay new from HongKong...
or you can get a regular 7" screen for about $175-$220 on eBay or a huge amount from the mo3car store that you will need to fabricate yourself like this:
There is also another option for a "transflective" screen for about $680 from the mp3car store.
music, gps, and video. A 700Mhz P3 or a via board of 1Ghz works very well. You don't need any faster. And these boards run cool too.
It is pretty hard to do it right, but easy to screw it up.