bmac,
Is the coop something that you plan on having for awhile? Or is this sort of an expriment that you don't want to put allot of money into?
What type of foundation do you have planned? Cement is the best, easiest to clean and the most secure to build on. If you do it yourself, it's also allot more work then wood and will require help. Wood is pretty common and should last for years. It will wear out and require maintenance that cement doesn't. You can build the floor on top of blocks, or sink posts into the ground. If you sink posts, you can make a small scale pole building. Do you have a post hole digger?
What were you planning on using for the exterior siding? Plywood type products are the cheapers per sq ft, but require the most maintenance. Mostly paint. The most commmon is T1-11, which some people like, and other hate. It's really personal taste. For higher quality and longevity, it's hard to beat James Hardi siding. You can choose between 4x8 sheets, or lap siding. The Hardi siding takes a few specialized tools, so that might be a problem. Cutting it and nailing it are the main concerns.
As for your roof, it depends on how you plan to cover it. Shingles or metal? Both have different requirments and skills.
For such a smal building, I would use 2x6's on top and a 2x4 on the bottom to create your triangle. Then I would use a 2x4 standing straight up in the middle. That will give you a very simple truss that will be plenty strong.
Don't forget to use headers with jack studs next to your king studs and cripple studs above and below your openings. It's the simple little things that people leave out of their outbuildings that cause them to fall apart so quickly.
Eddie