I haven't read the full thread, I will after this post and edit if needed
The prefabs are terrible!
My first one I thought was going to be larger than it was, it said it could hold 8. I was definitely disappointed once getting it assembled, it became my chick coop/daytime see-no touch zone, as I comfortably wouldn't put more than 2 full size and that still would feel wrong and small.
We converted a metal shed(8x10), aside from getting ventilation placed where it wouldn't affect structural integrity, it's been the best bet so far. More work to get together, but cheaper than wood.
We also have the
tractor supply XL sentinel coop, claims it will hold 14 and I have 4 bantams in it and I comfortably wouldn't put more than 1 or 2 more. It's not terrible with some work/alterations but it's capacity is WAYYY off.
All in all, anything you can find shed/barn or preassembled building is going to be the best bang for your buck.
Check local craigslist/Facebook/newspaper classifieds. People will get rid of them for a reasonable price, some even barter. So definitely look at all your avenues before you make a final decision.