July is peak season in most part of both hemispheres, December is painful to travel so I try to avoid winter/summer months during my international trips. Most part of the time, you will find that temperature will not be a big problem.
You should be able to pack your stuff in just one suit case. Most people will not stay more than two weeks on the road, and my trips seldom lasted more than three. Do some homework and be selective in your list, and always work in layers.
