From experience (and I'm no professional) I'd stay alteast a foot away from base of trees. Digging into the existing soil is no fun with all those tree roots, bringing in some compost and topsoil would be your best bet.. (your back and arms will be thanking you later). But for the most part we ourselves dig into the existing soil, use a mattock to cut thru the roots.
We've planted a few shade/woodland gardens under existing trees, we love this look. Hostas in my opinion get along with the tree roots and grow fine. As for azaleas, I'm not sure of, but if I recall I've seen it done.
I've heard some trees roots will find their way up into the new soil, but I've personally haven't had any problems with this.
Hostas will survive in pots throughout the winter.....as I have a backyard nursery and most will survive, occasionally you'll have one that doesn't but hostas are hard to kill and I live in zone 5.
Hopefully others will chime in and give their opinions.
