Your timing is good. They will be off of heat by October 1st and ready to head out to the coop full time. You will do fine without power in the coop if you have to but it is nice to have lights when it gets dark at 4 and have electricity to run a deicer for their waterer. Not the end of the world if you have to bring it in the house at night (they don't drink at night anyway) but it does make your life easier. You don't need to be concerned about them getting cold at night. Keep them dry and out of a draft, bed the floor with straw or shavings that will provide insulation from the ground.
Build the coop bigger than you think you need as they will spend a lot of time inside when there is snow on the ground. That you can get more chicks come spring,