Hi James
welcome to the forum. There is a great deal of experience and enthusiasm on here, and the forum is still pretty active despite the drift to faceplant. Other than some pretty dubious info given out on the FB sites, the other great advantage of the forum is that you can easily search for info, the vast majority of which has been asked before. Can't do that on FB. Also have a good look at the rides forum for some inspiration. It's actually quite easy to load photos by the way.
The inner wheel arch plate has long been unavailable, and second hand ones are rarely usable. But not to hard to fabricate a replacement. Rust is common in the rear pockets, bottom of front guard (due t those plates) and the front and rear sills. Also under the rear quarter windows. Really after nearly 50 years, they can rust anywhere. Well worth taking out the seats and carpets and having a good look.
VIN numbers didn't exist when these were built, so you rely on the body number. Virtually all after market parts are pretty poor quality, and the dash pads are no exception. OEM can still be got with a bit of hunting, but expect to be paying around $300.
It's not hard to retrofit a booster with twin circuit master cylinder. It's been discussed often on the forum. A booster doesn't improve braking performance, just how hard you need to push the pedal. Won't actually stop any better.
Spoilers I can't help with, but they were never available when new. Anything available is a generally a Bodge job and needs to be screwed to the body. Not really a great idea.
Webers are a topic of their own and a lot depends on the condition of the carb/s themselves. Then it's about balancing and tuning, and don't forget the general condition and tune of the engine.