The biggest issue with metal is that it "weeps" in winter and can contribute to frostbite. If you get metal, plan on lining it as soon as possible with some kind of insulating material to prevent this. But if you're thinking 8X8, ready-made fence panels from HD or Lowe's are easy to work with. I once made a quick coop out these for around $200, and it was easy to expand later by just backing the screws out on the gable end, moving that section back, adding two more on the sides, and extending the roof. I glued and screwed OSB (well painted) to the inside of the fence panels between runners for added strength/insulation. We lifted the panels off the ground about 6 inches, attached hardware cloth to the bottoms and buried it, lined the inside bottom with landscape timbers then backfilled inside with road base and sand to raise the coop floor, prevent flooding, and promote drainage. We set the fence posts to come above the panels by 12", set 6" "top plates" on the outside tops, and mounted the rafters to that. We filled in the gap with more hardware cloth and left it open for upper ventilation. The whole thing was trimmed out with 1X3 utility boards for a more finished look. I cut hardware cloth "windows" in the gable ends for additional summer ventilation. We also made the door "dutch style" (using part of the front panel and an extra fence post) and inset a large hardware cloth area on the top half. We mounted nest boxes, put in roosts, and the birds were happy as could be.
