i think trying to get the car to start if putting it aftermarket/standalone will be the biggest hurdle mate due to how the secuirty/imobiliser systems work now a days.the cars i work on everyday (not bmws but im sure there the same) even something like the radio/sterio being wronge and it will not let you start the car. Im sure it has been done but never really looked into standalone systems on new cars.I think youd have to somehow use the same wiring so as your getting readings to the oem ecu?