You're right...you will get a HUGE range of responses on this one. I DO use a heatlamp in the winter time (my coop is insulated). Some folks will ramble on about how it's not good for them, bla bla bla... That might be true if they were going from a balmy 80 degrees out into 15 degree temps, but nobody I know does that. Even with insulation, the highest I remember it getting in my coop last winter at night time was the high 30s (my goal was to keep it above freezing). My dogs and cats go from 75 degrees (in my house) to 15 with no problems, so I figure a chicken can handle going from 35 to 15...lol. Some folks will ALSO mention that they had a few little issues with frostbite on combs or toes, but it was really not a big deal. It WOULD be a big deal for me though, if one of my birds ended up with frostbite.
HOWEVER, I would be hesitant about using a heatlamp in a small coop, just because it doesn't give a lot of clearance for safety. Is your electric water heater going to be inside or out? Because that would put off a little heat I'd think. I used a homemade clay flowerpot water heater last winter, and tried it in a small temporary coop I'd make...it made a huge difference in the temperature in there...enough that I had to go down to a 40W bulb from a 60. It sounds as if you have a good plan in place for winter time. Your windows will help a lot too. Maybe next summer, after you make it through this winter, you can add some foam insulation to your coop??? It really does ease your mind.