Note: This is for the original 6 series. Not that monstrosity BMW is selling now.
I have never experienced the factory radio in a six but I can't imagine it has much to offer. As for replacing it the hole is a standard single DIN, the wiring is nothing close to standard. The factory amp was 2 channel and I bet yours is 4 channel, right? Well here are my suggestions for pulling of a clean installation. In convenient PDF format.