First with AT&T there are two (2) types of Free phones that are currently being discussed.
1. Free basic feature phone for
migrating customers ( somewhat limited set of devices ) You get a new AT&T SIM and a very basic phone without starting a new AT&T contract on a current AT&T rate plan without a contract. You only pay for the new monthly service + taxes + fees.
2. Free phone that requires a new 2 year contract! ( Note: this is not really a Free phone but a sales gimmic...

)
You are interested in option 1 only, the Free phone that does not require a new contract! Since you are currently out of contract, I would suggest you want to stay that way until the next iPhone is available.
There is no need to get a data plan now unless you are able to make use of it with your "Free" phones until you upgrade to the new iPhone. You will always have the option to add a valid iPhone data plan when you upgrade to the new iPhone.
As long as you don't have a contract, then when the new iPhone is available, you will qualify for the best pricing ( aka full discount pricing ) on the iPhone. Again only accept the discounted pricing if you understand that you will be starting a new 2 year contract with AT&T to get that discounted iPhone pricing.
Note: In addition to calling Customer Care, you also have the option to go to a Corporate Retail location and complete your transactions in person. Just use the same caution about what a free phone is or is not and make sure that it does not include a contract until you get to the iPhone.
Dave