Tern is fine quality. In fact, started by the son of Dahon's owner David Hon (Joshua), so there's some family lineage.

As far as derailleurs, the Shimano Tourney is definitely low-end but gets the job done. Keep in mind that when manufacturing a folding bike, much goes into the frame and in order to keep a certain price point, need to use cheaper components. Derailleurs can always be swapped, I wouldn't worry about that.