First, an iPhone 5 does have an internal A-GPS, assisted GPS (as do the cellular iPads, but with one important difference). The position is derived from two sources, first the built in GPS chip, second assisted by whatever is available. If your cellular is on, it will use the signal as assistance and improves especially first fix dramatically (remember GPS satellites do not send a continuous signal, so first fix with only GPS may take a while).
Second, yes you can use it as moving map, if you have sufficient maps on board. This can either be an online version (not very wise due to trouble if you loose cell connection) or an app with in-device maps. If you use online maps, yes, it does cost you data bandwidth and money according to your mobiles plan. Most prominent in aviation are offline capable solutions things like Skydemon, Airnavigation Pro, EasyVFR etc etc.
Now one more thing. If you put an iPhone or iPad in Flightmode, this will switch off your GPS, as it sits on the same chipset. This is where the big difference between iPhone and iPad is: at an iPad you switch off WLAN, BT and cellular data and the device is totally cellular silent, at an iPhone you cannot do this as the emergency telephone function stays active and pings to cell towers frequently. So, if you use an iDevice and want to comply with all rules, go iPad instead of iPhone!