Hi iron, I was told by my Vet not to use shavings for my chicks, use straw. I cannot remember why, but something about them getting sick. I always use straw for my babies, not sure what anyone else will say.
Also you are starting out fine with your coop and nothing wrong with that, just add as you go along.\\
If you come across a piece of clear plastic, or even the plastic they use for carports, you can add a skylight to let the sun in on the chickens. Not teasing you, my coop does have a skylight.