1. If you buy through Apple, you may not need a plan at all. It may be sold unactivated, and it might be possible to use pay-as-you-go (prepaid) SIMs from AT&T, etc., but we won't know for sure until Friday (or until more information is released).
2. You'll never pay for the entirety of a plan up front. You'll have a credit check, and you'll start getting billed for whatever plan you chose monthly. There is nothing magical about it.