1. For weather, you can see the forecast at Hong Kong Weather(https://www.travelchinaguide.com/climate/hongkong.htm) .
2. No, just bring with you some sweaters and jacket would be ok. If that's not enough, you can buy everything here.
3. The best place for electronics are the chain stores in Hong Kong, that's the most reliable places, also you can go to Mong Kok Computer Center, but there you need to clarify if the goods are good or not yourself. Some famous chain stores like Fortress has branches all over Hong Kong, you can get to the official website of Fortress for the prices and products information.

thanks