Well, since no one on the forums seems to know much about apple trees... lol. I have continued doing some checking. As I thought, apple trees are usually grafted onto a root stock. Grafting isn't hard but... you'd have to have some available root stock. Which, I don't know what it is yet.
But, I did find a place that sells 7ft trees (grafted, I presume...)... that's about where I'm at... at the moment.
Here in the south, I have successfully grown avocado trees from seeds (they're also better grafted). I just root em, plant em, water em and put them in the ground when they get about 4 foot high.
I would bet that your young seedlings will need to be protected from winter... maybe overwintered inside a barn or something? I dunno.
good luck... if I find out more, I'll post...