I am in the middle of this process at the min. I have the pc installed inside the case of my 6cd changer in the glovebox. I have replaced the oem screen with a CTF700h and am awaiting delivery of an RGBs to composite converter to reconnect the oem signal to the composite reversing camera input on the new screen. I plan to build a cmall circuit to add a simple push button to the car to switch between the carputer and the oem idrive and also connect it to the reverse for camera / sensor image from idrive.
Since my car is an E60 i have the Most bus, not Ibus, I have installed an E46 Ibus steering wheel in place of the original wheel, and plan to coinnect the wheel directly to Rolfs Ibus controller in my carputer. This should allow me to independantly control my carputer using Ibus without bothering the rest of the car.
What E series is your car?