I've heard it can get as low as -20C,
Sometimes lower.
Do people avoid heading out at nights due to extreme cold, or is it tolerable providing you have the right clothing?
It's not always freezing. Last winter was quite mild. I would say some people stay at home when it's bad. I don't though.
wondering whether to purchase myself some warm clothing before flying over,
You'll only need a warm coat, sweater and some good footwear.
It can get cold, but at the same time the streets might be dry with no snow.