Pine is most likely the least expensive wood to build with, at least it is in our area, and is perfectly safe for a coop. Lots of people avoid treated wood because years ago it contained arsenic to repel insects. They have changed the formula now though. It is usually treated with a copper solution now that is safer. You probably don't want to use it on the walls where the birds might peck at it though. Plus if you plan to paint your coop, treated is not a good option because it may take several months for the wood to dry enough to paint. Some wood is heat treated and is completely safe to use for most applications. Just about any wood safe enough to build a human house is safe for your coop.
Depending on how high off the ground your coop floor is, you may not want to cover or line it with anything. If it isn't at least 18" above the ground, covering the floor will not allow it to breathe and it could rot quickly from ground moisture rising up. With 6" of straw on top it won't be able to dissipate any moisture that gets on top either. If it is a raised coop, you can put down linoleum or just a couple coats of gloss paint to protect the surface and make it easy to clean. You can do the same for the poop board.
