I've done this both ways, I'd use the original loom & swap plugs on the engine side as you'll usually end up changing a lot of those anyway.
Series 1 is usually non ABS which requires wiring which I can't remember how to do as haven't done it for over 15 years.
The auto inhibitor is easy to bypass or in the meantime you can put it in neutral & unbolt it & tape it somewhere in the bay haha, then just wire reverse lights.
I'd go straight to aftermarket ecu.
A few basics are built in ignitor coils & reverse the CAS wiring, sort the plugs for whatever injectors & IAC-you can use an adaptor for the neo type otherwise the s1 will still work, use the knock sensors that suit the loom & it'll be pretty much running.