It depends on how you are using it. The absolute essentials are weather protection (sometimes this can be extremely minimal) and good predator protection if you lock them up at night.
You need a roost unless you have Silkies that don’t roost (some Silkies do roost, some don’t). Most (but not all) of us have nests in their coop. Some people feed and water in the coop, some in the run, some do both.
I consider good ventilation critical but so is keeping the floor dry or drying it out fairly quickly if it does get wet.
It’s hard to say what is absolutely essential because no matter what I say, someone will not have it and will do fine.