I have just that system in my 540. The head unit was enough power so I ended up hooking up a JL 500/5 amp, 10" sub and Focal speakers all around.

I don't know what mounting brackets are around for the x3 but the difficult thing was getting it in and not looking like crap.

The N3 is nice, I seemed to remember liking my Kenwood nav in my last car. The Pioneer is nice because it's all in one unit, no seperate brain box. I also have the ipod & sirius hookups. Both work great. No need for a changer.

If you consider a double din unit check your debth first. They may fit high & wide but debth can be an issue.