Take the advice with a grain of salt, but all folding bikes are supposed to undergo some fatigue tests and the Japanese have safety standards on products. I find it unlikely you would find a dangerous bicycle sold in Japan. The country is too well organized (difficult to bribe people) to allow complete junk to be sold in its market. I'm sure the Arun is made by a reputable mfg in China or Taiwan so although it will be heavy and not as refined as a Brompton at least it won't explode on you. You can't expect much from a 8500 yen bike, it's like a Walmart bike.

Maybe there's a popular classifieds you can find for a used bike deal?