I live in SW Washington in the Willamette Valley and I only provide a 100 watt bulb when it goes significantly below freezing for my Orpington shed coop. For my bantam coop, I am going to run the light bulb(s) in there for under 35 degrees or so. (I have two 100 watt bulbs I can turn on if needed.)
If you are going to be using a space heater, be very sure it is going to be a fireproof setup. I would NOT recommend a space heater in there actually, since chickens will knock it over, put dust, feathers, shavings on/in your space heater etc.
You need to avoid having extension cord connections out in the rain also.
Unless you have tiny bantams like me (Nankins and d'Uccles) I would recommend that you try them with no heat at all to see what works for you. Silkies are another one that I always did baby and gave them a light bulb since their feathers don't shed water.
The large fowl really don't like the light bulb on at night- it irritates them and makes them grumpy/peck each other. So I only do it when I feel the weather is just brutal.
They do make a "sweeter heater" for chickens that may be safer- I don't know.
If you do use a light bulb as I do, make sure that you hang it at least two ways (I use metal wire) not including the clamp, so a chicken could theoretically land on it and it won't come down. I have mine on timers.
Overall the greatest challenge is to keep the coop from getting moldy. I battle that every single year and sadly lost chickens last year since I threw feed on the ground, which got moldy and the coop got moldy since I didn't bleach it enough. They had that gasping/droopy wing and neck/quick deaths thing.
So make sure you keep on top of any mold is all I can say and keep fireproof the #1 priority.