A horse doesn't just run around in its pen to exercise. A horse will need to be exercised by you daily, or at a minimum every other day.
In the snow, there are a lot of things that you have to think about. Horses can get ice balls built up in their hooves which can make it so they can't even walk! You have to take preventative measures in that regard. Your horses will probably need blankets. Heavyweight turnout blankets will suit you, and not a cheap brand, either. You'll need to make sure that they're waterproof, breathable, and at least 600 denier.
Your horses will also have to have a shelter – preferably a very large, indoor enclosure, where they can get out of the weather and have plenty of room to move around.
Make sure that you consider all aspects of horse ownership. It's not a small thing!