I say tentatively, you don't need a device right out of the gate.
Depending on the app your building, you can do a substantial amount of testing right in the simulator. However, many features do require a physical device to text (ie. location/maps with movement, accelerometer, taking a picture with the camera, etc.).
I teach iOS programming, and I always tell my students they don't need the paid Apple Dev account until they have an app essentially done, that they want to publish. No sense in wasting $100 until you finish an app and know it will be necessary.
But, when you ultimately do get a device, I would get the latest iPhone. For one, the screen dimensions are different than all earlier iPhones, it has the latest hardware, and most users currently are ordering this device, as it is the new base standard.
Hope this helps.