[LS short box]

I would get the ECM from BP since you are getting the harness from BP. They can load a base tune to get you running.
BP can also include wiring for either one or two fans and also set the on off speeds.
Same for the AC.
Doesn't say what you plan for a trans. I suspect BP can supply connectors and tuning for all the popular LS type transmissions.
I would pick up a used mass air flow sensor for the same year engine.
Good luck with your project.