Welcome to the Forum, and get ready to be overwhelmed.. Literally thousands of helpful members, many with identical units and similar problems solved already! In addition, there are resources like Airstream Inc in Ohio, who can steer you to replacement manuals, and provide many parts. There are also specialized Airstream repair shops, like Inland RV in southern Calif, that stock old parts, offer advice and do repairs.
Furnace repairs, plumbing upgrades and A/C replacement are not terribly difficult, once schematics, routing and layouts are understood.. As you'll discover, many here are taking trailer projects down to bare frame, rebuilding the chassis, and starting from there to assemble a new trailer inside a refurbished shell...
The search feature is terrific, and you can look forward to lots of advice!