I'm wondering why did you decide to buy a used Dahon? Was the price really cheap? I've had a chromoly with a different frame and it developed play in the hinge from loaded touring. After 10 years my 2008 model became unusable so I had the hinge permanently welded together. The steerer tube fractured after 15 years. I've had an aluminum frame and it has developed a small amount of play too.

Although the frame that you have is a little bit more stable because of its pseudo top tube to help reduce the bending at the hinge, but without knowing the history of the previous owner I don't trust the hinges on used bikes. Just a little bit of play will gradually increase over time. So you might want to check to make sure there's no play and if there is, be careful with it and make sure it's adjusted tight.

Newer models have the steel cable from the head tube to the bottom bracket to help reduce the wear to the hinge. They cost around $500 at the official Hong Kong dealer and assume it's cheaper in China.