Honestly, Build it yourself. Buy a door (MUCH easier than building) Wood prices being what they are right now, go with a metal roof. Less labor intensive, too. The only thing that has going for it is that its easier to punch clean holes in than the plastic sheds.
I have yet to see a prefab shed (or any prefab chicken coops on the side of the road) that have adequate ventilation, and most of those that get close obtain it by putting windows at heights that virtually guarantee drafts on the birds, at least some of the year.
You can "almost" get "precut" by careful lumber selection (like 92 5/8" whitewood studs) - you don't think they are using PT wood in those, or even good quality externally rated OSB (OSB and External use is almost an oxymoron anyways). Use soil for the floor, invest in some decent wood for ground contact, and don't worry about bird beaks and mitered cuts for your rafters - this is for birds, not your next residence (I would recommend $4 for a plastic square and mitering your top cuts for the rafters, even for shallow angle - 3/12 minimum - roofs, but that's a handsaw and time, not a huge production. Work in terms of the available lumber (4x8 sheets), etc to further minimize cuts.
Dirt floor and deep bedding/deep litter to cut costs there, put the savings into spend elsewhere. Hurricane ties will ease rafter attachments and ultimately make it stronger than those screw thru designs they use in most of the prefab sheds I've walked thru.
Oh, and buy a prehung door, (I don't think I paid this much, but you get the idea) easier than building your own.