I just posted over on the duck forum about converting our old greenhouse for ducks, lol. We don't currently have anything left of ours except the frame so we're going to use welded wire and hardware cloth for the sides. Is it possible to do something similar with your greenhouse (remove some of the glass panels and replace with hardware cloth)?
For our chickens we use straw because that's the easiest for us to come by so as OrganicFarmWife said, use what's easiest for you to get and keep clean. I would avoid the sawdust because of it being so fine. As for roosting, you'll want to use something like a 2x4' board up off the ground, higher than the nesting boxes. Put the 4" part flat down so the chickens will have plenty to balance on and they can cover their feet when they sleep.
Good luck with your new flock, what breed are you getting?