Hi Ophelia,
I live in northern minnesota, and it gets cold here too.
First thing is that usually, frostbite is a problem for hens with large combs, and roosters (roosters don't usually tuck their heads under their wings at night, and doing that protects the comb).
Second, frostbite is much more likely to happen when there is too much moisture in the coop. You said you have good ventilation, even so you might try to increase it, even it it means losing heat from your coop. stick your fingers under your hens feathers, you can feel how warm they are without heat. You also might want to gradually lower the heat you are providing - I don't provide heat except when it is -40F or so air temp, not windchill. And then it is just a heat lamp over the roosts.
Frostbite usually doesnt bother the chickens - just watch the combs. If there is pus or wetness, you might have to do something, but otherwise it will just clear up on its own. I've had roosters with frostbit combs, the frostbit part just gradually turns black and falls off.
When you get your chicks, if they are day old or so you don't need to quarantine them. You do need to keep them safe from your flock - if you were bringing in older, say a week or more, you would want to quarantine them. strict quarantine is keeping them totally separate for 30 days, and then adding one of your current flock to the quarantine group for a couple of weeks. If your current hen doesn't get sick from the new group, then you can begin to integrate them.
You can also have the chicks you get vacinated....there is a ton on byc here about chicks, about quarantine, and about frostbite! lots of different opinions, so it is interesting to figure out from what you read what makes sense to you. good luck
