There is no data on T-mobile PAYG (except for the sidekick plan)
The only way to get (semi) PAYG data is on the AT&T GoPhone plan if you pick their hybrid prepaid solution.
You really only have 2 options:
1) Try and activate the iPhone on an AT&T plan, and enter a Social Security number in all 9's. This will fail the credit check, and you should be offered the AT&T GoPhone iPhone plan (ONLY on the non 3G iPhone). You are however, looking at a minimum of $49.99 a month, but you can end it any time you want, or simply stop paying.
2) Pick up any GoPhone from an AT&T store and activate it (once again on the Pick Your Plan hybrid PAYG plan). The plan starts at $29.99 for just 200 minutes. You can then add the GoPhone MediaNet plan for $20 (somtimes it can be found for $15), and then pop that SIM in the iPhone.
There is NO true PAYG data in the US, as insane as that may sound
Porting numbers on PAYG is tricky, it's all up to the operator whether they want to release the number or not.
Another option is the T-mobile Flexpay plan; this is essentualy a post paid plan, with prepaid requirement. You can pick almost any T-mobile plan and service, but you need to pay everything upfront.
https://support.t-mobile.com/knowbas...ic/tm22546.htm
I am pretty sure you can signup for Flexpay without an SSN or credit check, and the plan is then true month to month. Stop paying, and service stops, but once again you'd be looking at a minumum of $50 for voice+data.