You need the loom that matches the externals and sensors, and they all go in the head. So pick on that matches your head (c34 stagea?)
Having said that, looms can be modified if required.
You really need to change the ECU when you change to a 3l bottom end because the airflow will be very different, so buy an ECU that fits into your engine loom (although again, the loom can be modified for a different ECU, and many ECUs are total wire in anyway)