We're thinking of visiting HK on July 3-7, 2013. Is this a good time to visit?

It is OK, but the best time. It is hot during that time. The average temperature is around 27 to 30C. Thunderstorms usually occur, so it may be a little bit humid.
Summer clothes are enough. If you mainly plan to shopping there, you may need to take a thin coat or a T-shirts, because the air-conditioned works well in the shopping center.