unless your gonna move that tractor all over the yard. . I'd build one.. if your only gonna get 3 chickens it's big enough.. but you know 3 is probably not enough.. pretty soon ya want few more...it's up to you.. buying one would save you time... I build my own. .it's not that hard
Remember they kinda overrate size.. you need 4 Sq feet per bird in coop..so if that coop is 4x4. It's only good for 4 chickens.. the run recommend is 10 Sq feet per bird.. so just keep that in mind
You don't need to insulate. . Long as it's enclosed to block wind..
Far as a setback..I don't think so. It's considered a small shed.. I would pay attention to how it affects nieghbors..
Is it away from the nieghbors bedroom window? Chickens make noise in mornings.
Will snow from your coop land...