You are better off buying the unlocked T-mobile iPhone from Apple. It will cost you full retail price. It would be the same price at T-mobile, but they will give you a phone that is locked and you will have to go through the hassle to get them to unlock it a minimum of 60 days after you start service.
As far as the service plan, since it is prepaid, there isn't any need to bundle the plan. It's not a family plan.
You will have to get 2 SIM activation kits from T-mobile. These are often had at a discount from their website....as low as 0.99. They don't have SIM kits with micro SIMs, but don't worry....just get the full size SIM ones. All you need is the activation code inside. Make sure you got iPhones with T-mobile SIMs in them from Apple. When you activate, just use the number on the nano SIM that came with your phone with the activation code that came in your SIM kit. You won't use the actual SiM card that came in the kit.
Make sure you sign up for the right $30 plan. If you don't choose that plan at the start, you can't switch to it later.