Everybody's walk line is going to be different, and it may even vary from trailer to trailer for the same person. My personal walk line was that I wanted to avoid doing sheet metal work, but the trailer I bought ended up needing a complete shell-off, and I have replaced a flat panel and a formed segment--it is a slippery slope. You end up discovering that none of it is rocket science, and anyone can learn to do the repairs needed.
Recently, I have seen forums members taking on projects that seem a bit desperate--trailers that have been crushed by falling trees, etc., I would definitely walk from one of those--but others see them as a challenge.
Long and short, do plenty of research on the forums, buy the back issues of the "Vintage Airstream Podcast," and you may succeed in calibrating your expectations. There are a lot of trailers out there in varying states of disrepair. I don't think they are so hard to find that rebuilding a destroyed trailer makes sense. But that is just me. If you want to spend your time camping, rather than rebuilding, then look for a trailer that has already had the "heavy lifting" done. Any trailer that is 50 years old, and has never had any renovation done to it will need a lot of work. It is all a matter of time and money. If you don't want to spend the time fixing it yourself, then it is going to cost to get someone else's work. The bargain priced, all original 50 year old trailer that can go camping with a little sprucing up is a rare animal indeed.