In a related question I asked about the functionality of an iPhone 6 as compared to iPod touch 5, The answer indicates there could be a significant difference.
The person I am asking on behalf of is currently using a tracfone for cell service and paying $80 per YEAR for more service than she uses. It seems possible to purchase a iPhone and not get a data plan and to use old phone as an development device Apple even posts some about using a iPhone 3 without a plan. I found an older post about switching SIM cards but I am not seeing anything about purchasing a brand new iPhone 6 to use as an iPod without a contract. There are some options to use an iPhone on tradfone but again only with old phones that have expired contracts.
Best Answer
Yes that works fine, just know that you need to pay full retail for it so you are not beholden to any carrier to pay for the rest of it. So basically you are talking about an unlocked phone.
The Apple salesperson should be able to help you get what you want along with advice on how to set it up.