There are no computers in the the sils of the car. The lowest computer is the air bag computer which is on top of the tunnel under the ICC. I am not aware of any issues with starting the car after the airbags have been deployed. I removed all the airbags and the airbag sensors from the 540 and the only thing it does is brings up SRS fault codes in the system. The seat belts also use this same computer, but again no issues when starting the car.
All the computers in the system (including the HIM which controls the air con)need to be connected up for the car to start. There are no indepentant computers in the system.
As far as the wiring loom goes(Sideshow might be a help here) I would use as much as the original loom as possible and strip out what you don't need. There will be "hand"wiring required for the dash.
Talk to Peter R regarding who to use regarding turning off unwanted computers in the PCM. There have to other companies that can do this. I know Herrod Motorsport(Melbourne) has "broken" into the ZF programing and is the Australian agent for SCT flash tuners. I don't know of any local companies that have the ability to play with the PCM programing. If you find someone let me know as I have a couple of things I won't to sort out in the 540.
Warren