Dig a test hole before you buy your posts. The deeper you can set them, the better. Hope for 4' deep. My bedrock is only 2' down where I set one pole in concrete, and it needed substantial bracing. A deep hole is better.
Kind what he said. If you use wood get it below frost line. Where I live its about 2 1/2 feet to be up to code for load bearing structures (decks etc) but we rarely get snow or really cold weather.
If you angled the posts outward would that do anything to lessen the stress on them?