As far as I know, the iPhone's consistently have some of the best cameras for well-lit environments. That said, depending on detail needed, most of the Etsy folks I know insist on a dedicated camera, tripod, etc. Not sure what you're photographing.
The exact carrier depends on your usage pattern (and willingness to change it). I use the phone so seldomly that a pay-as-you-go plan with AirVoice is by far the cheapest option for me. AirVoice and P'Tel are my two favorite recommendations (via I.P. Daley). Ting has both CDMA and GSM coverage. The GSM coverage is via T-Mobile, like P'Tel. AirVoice is ATT-based, which is why I picked them, as T-Mobile's coverage here is too spotty once into the ex-urbs, which I travel frequently enough to need the advantage. Pricing is all pretty similar but will vary +/- a few $ each way depending on your exact usage pattern.
Both are GSM-based, so any unlocked GSM phone will work. For iPhones specifically, I'd probably insist on at least a 4S or newer. (Note: 5C is essentially a reskinned 4S, vanilla 5 is better, then there's the new 6/6 Plus). iDevices are repairable at many places, whereas the repairability of Android phones is a really mixed bag (best being Samsung Galaxy devices, worst being HTC phones, etc).
For acquiring, others may have better recommendations, but I'd choose either a highly rated ebay seller or one of the 3rd party resellers of used phones like swappa, glyde, or gazelle. It's also worth asking local friends if anyone has an unused GSM phone laying around. Many don't bother to resell their old gear, and 4S are past the 2 year upgrade treadmill window many contract cellphone users put themselves on.
Hope that helps! Feel free to ask clarification questions.