I'm interested in visiting China (Great Wall & Hong Kong) early May, is that last still low season?

It is advised you go to Beijing to visit the Great Wall. There should not be too many tourists in early May. But you should avoid the May Day Holiday at the beginning of the month.

The weather in Beijing is warm with a temperature of 60~80F while Hong Kong can be a little hotter at 75~85F.

There is one direct train at Beijing West Railway Station to Hong Kong, departing at 12:40 everyday. And you will arrive in Hong Kong at 13:00 the next day.

You don't need to worry about the accommodation, for you can easily find many chain hostels and star-rated hotels available in the two cities. In Beijing, you may even consider staying in the international youth hostel.