You are exactly correct that there are many decent quality used structures out there right now, in part due to the removal of signs by the largest outdoor companies. You can can find them in two places: 1) call the big companies direct and see what they have laying in their yard. That’s where most of these take-down units end up. I’ve seen stacks of them 8 feet high. Normally, if you are not a threat to them, they will be happy to sell them to you, since they have no where else to put them 2) call the major sign erection/fabrication companies in the major markets where the larger companies operate. They also have them laying in heaps in their yard.
In every state that I have ever worked in or even looked at working in, you must submit engineered drawings with your permit application, signed and stamped by a licensed engineer and guaranteed to withstand a certain wind load (wind speed). There is no way around this to my knowledge. Even on old, used structures, you’ll have to pay an engineer to draw up plans for its re-installation.