Farmer Viola, I would HIGHLY recommend against a space heater. We had a guy burn down his coop and house and scorch the neighbors house because his birds knocked over a space heater in the coop. I was able to get my 8 week old pullets out at the beginning of December and the temps were in the teens at night. I just have one heat lamp in the coop with a shatter proof bulb in it. I only use it on the coldest night/days.Just make sure you're being REALLY REALLY CAREFUL about using/securing/monitoring it. Over in the Michigan thread (and it's a whopping 9 degrees here this morning) the standing consensus is to avoid heaters, period. If you have an unheated garage or basement or breezeway/mudroom where the outdoor temperatures are moderated by the building I would put them in there for a week or 2. (I don't have any of those, so I understand how this can be an issue, and it's why I don't hatch between October and March.) I'm sure you've considered it, but it's always a good reminder, especially since a lot of people read but don't post.
I just got them used to the cold by turning off all heat sources in the room that the birds were in once they were feathered out at 6 weeks old, and then I started taking them out side for short periods of time until they were able to tolerate it for an hour. I knew they were ok because they were scratching around for treats. I picked a "warm" day to leave them out for the first time, checked on them frequently and brought them in that first night, the next day I left them out all day and all night, and they have been out ever since with no problems. Just do it slowly and keep an eye on your birds.
I could see myself creating a spider web of ropes to hold it up from 10 different angles... LOL