I'm a current owner of an AVIC F900BT and I'm planning on upgrading to the NEX-4100. I currently have a backup camera, Sirius, GPS, and microphone all routed through my car somehow, along with the Peripheral box to keep my steering wheel controls. What can I keep in place and re-use in the NEX4100, if anything?

you can keep all of it, you might even get lucky with the wire harness being the same.