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If you have a choice, go with a dirt floor coop--sooo much better! I do have a coop that's off the ground on posts and it seems satisfactory to the chickens, plus they do like the space underneath for hot days. But, I built it from very sturdy materials and @ 4' x 8', it's a lot bigger and stronger than mass produced pre-fab coops. I wouldn't go with a mass-produced pre-fab. If I were buying one ready-made I'd ask around and find a local who builds them.
Any coop you're likely to purchase AS a coop is likely only going to comfortably house 5-6 birds at the *most*, and that's not allowing for the fact that northern birds may need to stay inside for weeks at a time due to snow over their heads. A good alternative, if you can manage it, is to purchase a shed and convert it and add a little chicken yard to it.
Look in the "articles" section for ideas. There are many coop articles.