I would think buying and selling a used Airstream would be the least financially successful path to take. You would stand the chance of a long resell duration and might take more of a loss with the higher priced trailer. With your Honda Ridgeline and schedule of events, a pop-up makes the most sense.
If hard sides is a must-have, then a small used trailer is your best choice. There are a lot of vintage trailers that will offer a basic camping experience and many of them are easy to maintain and fix.
Good luck with the hunt!