My coop is a section of a larger lean-to. Two sides/walls are hardware cloth, and the two solid sides have about an 8" high ventilation opening all the way around, between wall and ceiling. In the heat of summer, I still run a fan. And you are further south than I am.
The old farmers around here often provided only a roof, one or two walls, and nest boxes, if that. It was not unusual to just let them roost in trees. Obviously, that does not address predators, but I think you need to give some more thought to shade and ample air flow. Note what DigitS' said. Chickens tolerate cold much better than heat; cold is really not an issue for them down here. They do like to get inside out of a heavy rain, though mine are usually outside when it is raining lightly, even in winter months.
