Welcome to BYC and the wonderful world of backyard chickens. If you put your location into your profile people can give better-targeted advice to all your questions because climate matters.
Unfortunately, VERY few prefab coops/kit coops are in any way suited to actually keep chickens in and even fewer are well-built.
In your part of the world, however, you don't need a traditional enclosed coop at all. An Open Air coop is perfect for your climate. That is, a big wire box with a roof and a 3-sided shelter on the windward end.
Here are some Open Air coops for you to look at:
https://www.backyardchickens.com/articles/jens-hens-a-southern-texas-coop.75707/
https://www.backyardchickens.com/articles/my-positive-local-action-coop.72804/
https://www.backyardchickens.com/articles/california-living.68130/
Also, my Camp Cockerel -- which is not a fully-secure facility, but the idea could be redone as a properly secure coop:
https://www.backyardchickens.com/articles/camp-cockerel.77789/
Some people turn a chainlink dog kennel into an Open Air coop by adding hardware cloth to keep smaller predators out (rats, weasels, snakes, and grabby raccoon hands). They put a roof on it and some kind of solid panels to shelter one end.
Hoop coops are considered some of the easiest builds for people who are new to DIY:
https://www.backyardchickens.com/articles/hoop-tractor.69336/
https://www.backyardchickens.com/articles/hoop-coop-brooder-with-roll-up-sides.75720/
https://www.backyardchickens.com/articles/the-biddie-bordello-a-hoop-coop-run-combo.72189/
https://www.backyardchickens.com/articles/permanent-hoop-coop-guide.47818/
Here is my article on hot climate chicken-keeping:
https://www.backyardchickens.com/articles/hot-climate-chicken-housing-and-care.77263/
And my article on coop ventilation:
https://www.backyardchickens.com/articles/repecka-illustrates-coop-ventilation.77659/
And some numbers (consider an open air coop both coop and run combined).
The Usual Guidelines
For each adult, standard-sized hen you need:
- 4 square feet in the coop (.37 square meters)
- 10 square feet in the run (.93 square meters),
- 1 linear foot of roost (.3 meters),
- 1/4 of a nest box,
- And 1 square foot (.09 square meters) of permanent, 24/7/365 ventilation, preferably located over the birds' heads when they're sitting on the roost.
4 hens
- 16 square feet in the coop. 4'x4' is the only really practical build for this given the common dimensions of lumber.
- 4 feet of roost
- 40 square feet in the run. 4'x10' or 5'x8'. 6'x6' is a bit too small, 6'x8' is more generous and easier to build than 5'x8'.
- 4 square feet of ventilation. A 2'x2' window is theoretically enough, but in practice doesn't create any air FLOW so better to spread the venting around (and even better to exceed the minimums, especially in warm climates).
- 2 nest boxes, to give the hens a choice
