For the coop? I would us plywood ... although particle board is slightly more dimensionally stable, it can warp more easily over time with moisture. The plywood is stronger and reasonably dimensionally stable. T-111 is common for exterior work. Paint or stain the exterior surfaces with something durable to outside conditions. I would only use the pressure treated wood for portions making direct contact with the ground or that are exterior, but not being painted/stained.
I'd say 1/2" is fine for walls, I'd want 3/4" for flooring and probably the roof as well
No clue about the paint ... they will tend not to pick at the outside walls, so I would not over worry ... more important, I think, to get something that will stand up to the outside elements. I would avoid oil base paints, but latex and acrylic paints are so much more easy to use anyway!
yes... there in lies the dilemma... Maybe we could go with a thinner one now and source used vinyl for later...
Also new construction in your area ... some contractors won't mind giving you smaller cut offs that they might not end up using.
