With a bit of imagination and a bit of money, you could transform that garage into a palace combo of coop and run.
Basically, you have the solid structure to start from. What I would do is look at it as a year-round coop and run and divide the space into a sleeping area that is enclosed and a run area where you will cut some nice big windows and maybe even a couple of sky lights for natural light and stellar ventilation. You can then add an outdoor open run later.
For bedding, I suggest construction sand. I had shavings and hated them consistently for the entire time. When I removed every last shaving and installed sand, I've never had one moment of regret. It's got so many pluses, it's practically a no-brainer.
Sand acts as a heat sink, making it easier to maintain a constant temperature. Sand is easy to scoop and keep clean and maintain an odorless coop and run. Sand is a natural dirt bath that the chickens enjoy spa time in, especially if you position windows to let in sunlight to warm those bathing spots. It's a convenient source of grit. Sand is easy and cheap. A dump truck from your local gravel yard can dump a pile in a strategic spot and shoveling it into the coop and run is a few hours of a physical workout, but it lasts for a few years before you need to replenish it.