Since your husband knows how to build, I think building a wood structure from scratch would yield best result. The shed could be used for storage.
Also as far as your concerns about open air set up, cold isn't an issue unless you get well below freezing. Most chickens are perfectly fine...
Your shed is way too small for 30. And yes I'm also concerned about the heat since it's metal. Is there anyway to expand it, maybe use the metal section to house just the nests and feeders maybe, and then build out the front so it's open air with a roof overhead. Sort of like this (but with a...