Load Bearing Wisdom

Big sheet of linoleum flooring will make it much easier to clean out. Put it 6-8 inches up the wall and cover with straw. Then shovel it out quarterly. Whatever linoleum is the cheapest, patterns shouldn't matter since it'll soon be caked in shit

In Maine you may want to look into a diesel heater or propane, since it's cheaper than resistive electric heat

I do not have chickens myself, but my brother does. Every 2-3 years, he picks up & moves the shed to another location, because the chickens shit constantly. Gets so bad he cannot keep up. You will not have that option, so stop & think in advance how you plan to keep the place clean. Depending on your tolerance for cleanliness, it may not be feasible to use the shed for anything else once you've had chickens in there for a few months.

Also, the chickens attract all manner of wildlife, from other birds who want to grab food, to snakes who want to steal eggs, up to downright predators who want to eat your chickens. Realize you cannot simply let them out in the morning & come back from work in the evening & expect everything to be alright.

I suggest ducks. Stink less, less noise, can be trained to come out of the coop to forage in the yard and return... The only real downside is less frequent eggs.

I have the link below bookmarked because I was hoping to do this myself. Good luck and keep us posted.

https://www.pamsbackyardchickens.com/2021/04/how-to-turn-shed-into-chicken-coop.html?m=1