Currently, our house has some boilers and radiators throughout the house and also we have a high-velocity cooling unit. I’m planning to do a Reno for an addition to the first floor and second floor and I need to find the right ones for the house

I am planning to keep existing radiators but for an addition would it make sense to add heat pump or use existing cooling unit with duck work and possibly upgrade capacity and add radiators in addition?

Concerned with the heat pump is how well can you function when it’s -20 outside like in Toronto and winter?