You will need:
-A retrofit wire harness
-The carrier bracket if you don't already have one
-A radio unit which is going to be mounted next to the battery
-The Nav screen and it has to be the one with the tape player in it, why? i do not know, even though there were already BMWs then with CD players behind the nav when they came out with the X5
-The Navigation Computer (MKIV) and brackets for that.
-The CD changer and brackets to attach it to the carrier or...Ipod interface
-Screen bezel, since you need one that is specific for the X
-Navi disc
-Navi GPS antenna

There is a write up somewhere here. But it will need a lot of time...