Hello,
Without really seeing the potential purchase, it is a guess and by golly! However, if the floor is "spongy" my guess is you will need to replace a portion (at least) of the floor. If the spongy area is near the water heater or other water line, then check the pipes to see if there were some broken. That would indicate lots of work, and expense.
There are some forum folks who will come and inspect trailers for potential buyers if you post a request. This can happen if you are in the same area as someone who feels comfortable doing this. Generally [I think] if you offer to reimburse folks for their travel expenses (gas probably) and maybe buy them dinner or something to thank them that is all people might expect.
There is a great thread (you may have seen it already...) on considerations for buying an Airstream. You might want to check it out. Lots of good information there on what to look for, etc. I think there is even a link to a printed list of items to review.
http://www.airforums.com/forum...eam-22920.html
Older trailers usually need some work/repair. If you are planning on having someone else do it for you expect to take out the wallet! Maybe a handy man/woman type friend would work with you on a project?!
Good luck! I would take a digital camera next time you go look at a trailer and then you will have references to things you forget about when you get back home. Really helps the memory factor!
Mrs. NorCal Bambi (traveling in S Tardis)