Going back to your original question, in vintage FGRV's it's all about condition, condition and condition. While my 1973 Hunter needed very few structural repairs, I have seen other vintage FGRV's with everything from broken door hinges to cracked frames.
That said, most brands of vintage FGRV's are fairly equal in quality, but a thorough inspection is your best friend. If you are unfamiliar with how to inspect a trailer you might want to find someone to go with you who has some experience with FGRV's or at least with RV's in general.
BTW: What will you be towing with???? Some FGRV's are a bit heavier than they appear and weight
is always a considerations.