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.