Growquest has a pretty horrific reputation on more than one of these forums. if you do some searches you can find some horror stories -- but you will have to judge for yourself how much is true and how much isn't. I wouldn't order from them myself, based on comments from long time posters whose judgement I trust, but again, you have to judge for yourself.

You might have better luck with a smaller variety probably, there are some that stay on the small (10') size.

I think you would have a lot of issues to deal with. For example, Avocado fruit takes a long time to develop and ripen--mine, for example, sets fruit in Feb-March and it's ready to harvest around Christmas/New Years. So you'd need a smooth transition from being outside to being in your sunroom to avoid fruit drop--issues like that.

I'm sure someone is out there doing it, but it might take more effort than it is worth. It's a subtropical plant.

I don't believe that the avocado tree is as fragile as some folks think it is. I remember my father driving nails into the tree trunk to "jolt" it into believing it was going to die. This seemingly Draconian act seemed to cause the tree to produce flowers which eventuated into fruits.