The posts were sunk into the ground 3' in concrete....so they would be in contact with the ground at all times.
We also used treated for the outside framing of the floor...they would possible be exposed to the weather.
Then inside that frame we used regular construction grade wood with regular 3/4" plywood on top. (To save on the cost of the coop)
We also used 2x3's for wall framing instead of 2x4's. We also added a primed siding product that came in 4' x 8' sheet.
I believe it is an exterior siding. it has fake grooves every 5" or 6" i think, with an over lapping edge. (just nail it up easy!)
The size of our coop is 16' x 10' (we have 40 chickens...lol)
You can use whatever you feel comfortable with. We tried to keep our costs down as much as possible. With that said it still cost a lot.
We did have some things already. but not much.
I did find a $13.50 pre-hung metal door with a window in it and 3 free windows from our local habitat for humanity resale store. I did a lot of shopping there. They have every thing.