Questions about switching to iPhone

I'm thinking about switching to an iPhone from my Galaxy S6. I would probably be getting an iPhone 6s or 7 depending on how much the price drops with the release of the new one. I have a few questions about the process of doing this.

1. I don't want to be on a payment plan with AT&T, so can I just buy one from another seller and still use it through AT&T? I mean I know I can, but more so how?

2. Do you guys have any reccomendations of where to buy from? What is the best price you think I could get?

3. Can I transfer my contacts, pictures, and other things from my Galaxy to the iPhone?