Buy the winter gear in Finland in stores so you can learn what is good. Stuff online from out of the country often isn't suitable, not windproof, waterproof etc.
Be careful where you buy online from. You can end up with high customs costs.
Yes there are plenty of secondhand places here called "kirpputori". And huuto.net is good. Tori.fi and keltainenporssi.fi. But huuto.net seems to be the most popular.
Look on Facebook for the foreigners buy and sell group. I can't remember the exact name. From that group you'll find almost everything you need cheaply because it's usually foreigners leaving. Though it does tend to have a lot of baby stuff too.
Do not overdo the winter clothing. It's actually more dangerous to your health to be overdressed than underdressed. You sweat, get wet and then cold. That's worse than being a little cold from being underdressed.
Shame you didn't let me know you were coming a month ago. I could have sold you everything you needed cheaply

At least house wise.
Welcome
Please don't die of a heart attack at the cost of living here lol