Most older trailers do not need the interior replaced. If the inside is realy bad you can repaint it. (check this site and others for the correct paint) 1950's and up to mid 60's had very sraight forward cabinetry. If you get one with the tambour doors it's a little harder to repair. New tambour is available, but pricey. The first tool you buy with an Airstream is a good rivet gun. Almost everything is riveted. Drill out the old ones, pop in the new ones.
One major problem can be the floors. If there are just a few small soft spots, you can patch it. If the floor is really gone you will have to lift the body off. Almost everything in the trailer can be repaired or replaced.
The most important thing is a good straight body. Dented panels can be replaced at about $1000.00 per panel. Getting the frame parts replaced is a little more.
Even the steel frame can be repaired if need be. It all depends on how much work you are willing to do.
What is your goal? If it's to have a 40 year old show winner, you've got a lot of work and a lot of money to spend. If it's just to have a nice, usable trailer then it's not nearly so much. In either case you're going to spend a lot more time on this project than you thought. To be honest, you will never really finish. There will always be one more little thing you'll want to do.