Welcome to the Forum, Deano:
Lots of questions for one post - I'll try to address them in order - keep in mind that my trailer knowledge comes mainly from my '78 Sovereign
of the Road that is undergoing a slooooowwww repair for mostly minor items.
1.) Most any Airstream repair facility will sell the preformed Al panels.
2.)Any appreciable amount of repair requires removal of the inter plastic endcaps - the endcaps break and crack easily, and I doubt if you will find replacements for them - handle with care!
3.)I do not think these items are off of the shelf - but they are easy to make since no compound breaking will be necessary.
4.)The framing and outriggers (at least on the '78) are lightweight - be careful what you weld for reinforcement - Spare tire carriers are generally thought to be a major mistake - search the Forums (search function on pull down bar at mid-top of entrance page).
5.)I do not think that the bolts need to be high strength - plywood replacement may be more difficult than at first glance, since the plywood MUST be inserted between the shell "C" channel and the frame. Your Airstream is a monocoque construction, and the shell, floor, and frame work together for structural integrity....a strong (no rot) floor is absolutely required around the entire perimeter of the trailer....the inspection of the full perimeter is a necessary pre-purchase inspection, but usually entails a considerable amount of cabinetery, bedding, and seating removal to do a proper job of looking at the perimeter.
6.)Replacement depends on your peace of mind - how much do you trust 30+ year old gas controls?
7.)Most of us here in the Forums use PEX, or a similar product for water line repairs - your preference - most of the time PEX is easier to install than copper, but not always.
8.)New tanks are also your call, I usually carry bottled water for drinking and cooking, and almost always camp where there is water available, so the main use of the potable water tank is flushing the toilet when not hooked up to campground water. I would not want to drink water from an old plastic tank - bugs and other such nasties. The water pump, if it as a PAR, should have replaceable parts, and most of these parts are readily available.
Good luck - let the Forum know how you handled your situation.