Agreed, wood will be much easier to work with, modify, and will last much longer. Won't degrade in the sun. You might find some shed models at Lowes/home depot that are being replaced, and get one at a discount. Also, check for private builders who build storage sheds. You might even be able to contract with one to modify one to your specs as he builds it. An other option would be to contact your local tech school and see if there are any students that the teachers would recommend to build your coop.
Check Craigs List too, if a house is being redone, they want the shed gone.
FYI, I have the Rubbermade shed because i am only going to be here for 2 years then hopefully finding a farm where I will build a wood shed ! This way I can reuse this shed for a shed later And it will come apart so I can take it with me