Finding the right folding bike should be tailor made to the individual(s) needs and desires, both present and with future growth potential built right in. I did this for my and my family's own folders with 99% accuate anticipation. My bikes are rather simple in design and approuch. I purposely purchased the entry level or budget models of Dahon (Boardwalk S1 converted to AW three speed) and a Brompton C type. Both were customed ugraded to what I wanted, rather than what a bike designer thought I should have.
For the type of riding and terrain that you will find in Mexico at that time of year might approximate my own bike's features. You will be happy with a wide variety of folders. The used market may offer what you want-then again it may not. I no longer buy used since the technological development of the folding bike goes foward rapidly and the price range dictates a more satisfying bike offered new than used (more choices, warrenty, and price more reflective of it's true value-not an inflated one. Just don't rush in and buy just anywhere. If you are new to the folding world, stick to a bike shop rather than on-line. Go to shops that carry at least one known brand (Dahon, Bike Friday, Brompton etc.) and try out the bikes. Then decide.