Here is his cage.
Obviously you don't need one that big though. Just an example. Birds are much happier the more similar their cage is to their natural environment.
I have a friend who breeds cockatiels. I will find a picture of his cage and post it on here. His stay outside all year round. He has an indoor section where they can go. You could possibly hang a heatlamp up during the winter.
Cockatiels will need a much bigger cage than a budgie because if their size and wingspan, therefore costing more. Also, the bigger the bird, the more food you'll get through.
I always think it's cruel to keep cockatiels inside.