This is a low end derailleur but should work just fine. My wife has one on her Nova with 1500miles on it and no problem whatsoever. If it will go bad, you can replace it for less than $15. Note that there are several versions and you will need one that fits the hanger on your bike.

As for dahon vs tern. Until recently there were two separate companies using the Dahon name, Dahon China (father and uncle) and Dahon Taiwan (son and mother). Dahon Taiwan is now using the brand Tern and gave the dahon.com domain to Dahon China. I would expect tern to but just the same quality if not better.