ECU is in passenger side footwell.
Take the dizzy apart, make sure the CAS disc inside is clean.
I had misfire issues because the CAS was rusty and spilling dirt on the disc (sensor wasn't reading properly hence misfire).
Also does it idle fine? Try unplugging AFM, it should idle and rev to 2,000 rpm fine.
Loads of info on R31 Skyline club wiki
Hopefully it doesn't fall apart. But probably fine for everyday driving. Looks like it's used by a few Nissans.
Not sure what AWD will do when you're aquaplaning, but I'd never skimp on tyres (or brakes for that matter). I've learnt the hard way.
What issue are you having? Seems pretty straight-forward, just need to move the wipers to 'service' position, then pull off the wiper (J hooks are easy to deal with).
Throw some Bosch AeroTwin or whatever they're called these days, and don't worry about it for years.
I bought the same HFM BM57 copy master cylinder and my rear brakes didn't work (reverted back to standard BM50).
So yeah, see if you can put the standard master cylinder back on and test.
That's pretty easy to clear up. As said already, get a headlight restoration kit or get a pro to do it for you (pretty easy to find on Facebook).
I found the Philips kit pretty good. You'll also notice improved headlight output as your lenses are clear/transparent.
I was thinking about this meme the other day
Also, welcome OP. What are your plans for the car? They're easy to modify so you should have plenty of fun with it.