Specific coop

Howdy, all,
New to chickens and lovin'it!
I have 3 two-month old "pollitos" (chicks) that are currently overnighting in a large cardboard box in my home. I want to build a coop, but being new to these animals I am unsure how to proceed. Let me give you info and hopefully some of you would be kind enough to furnish information in return that will help me to build an appropriate "home"for my birds.
We live in central Mexico above 6,000 feet altitude. Winters usually consist of several hard frosts in January/February.
Spring comes in March, and April/May are darned hot. Rains begin anywhere between May and July and last about three months. It never snows.
I intend to build my coop inside my barn, which remains dry but is subject to lots of fresh air. I have the usual suspects as predators to include skunks, snakes, possibly rats. Because of snakes, I will not be using chicken wire. My birds are for pleasure and fresh eggs, but not for meat. They will be free range birds during the day. They currently terrorize my garden, but as they grow older and larger, I expect they will go further afield. I wish to include a roost area and an egg-laying area, and all easy to clean. I am most unsure about how chickens are affected by drafts and such. Most coops I've looked at seem like they would be awfully hot - not much provided for in the way of fresh air. My idea is that I could cetainly put up and remove plywood as the weather dictates.
Any help/information you could provide would be greatly appreciated.
Thanks!
Moonunit