Welcome to the Forums.
If you go to the Portal page, you can download the "Buyer's inspection checklist" This will help you to do a thorough inspection of the trailer. Don't take the seller's word for anything. They all think their trailers don'e leak and have no floor rot. You may also be able to find a volunteer inspector in your area (also on the portal page) who can help you see the trailer for what it is.
As to value, if the floor is solid throughout (especially in the very rear), and the frame is solid too (very hard to determine just by looking at it), and the trailer has no dreaded "rear-end separation" common to the 70's era trailers, then $5500 probably isn't too bad a price for it. If you have been looking for a trailer for very long, then you should have an idea by now a ballpark of the "going rate." How does this one compare? If you haven't been looking long enough to get that figure in your head, then don't get impatient and settle for something that isn't just what you want.
Just realize that when buying a vintage Airstream, especially one that is over 40 years old, the only work you won't have to do is the work that has already (recently) been done. So plan on repairs of every description (plumbing, electrical, mechanical, suspension, upholstery, etc.). The $5500 purchase price will just be the down payment, as the costs for rennovation can add up quick.
As to "devaluing" the trailer, once you buy it, it belongs to you. Do with it what makes you happy. When it comes time to sell it, even if you find a potential buyer who turns up his nose because is isn't and "all original" museum piece, you will likely find two or three who will appreciate the modernization you have done to it (if you do it well).