Here is the lowdown on iPad flying:
You don't need the one with the large memory, but more memory allows you to put more of other things on there, like music and movies so you can use it for other things too. The ones with 3G do have built in GPS chips versus the ones that do not. However, it costs $129 more for the 3G model of the same drive capacity. An external GPS has on average been more better at retaining a signal and costs $99, less than the $129 to get it internal. The external GPSs come in two flavors, a plug in one and a Bluetooth one. If you go with an external you need to decide whether or not you want to remember to unplug it before you bump it into something or remember to take the Bluetooth one with you when you leave your aircraft. I solve this problem by lanyarding the GPS to my 9G kneeboard clip. When I go to leave the aircraft I make sure to reattach the GPS to my kneeboard so I don't forget it. This works for either model. Also, the Bluetooth one has it's own internal battery you need to keep charged. I use the plug in one by Bad Elf. I say get the wifi only model and the Bad Elf. It is a better GPS and a cheaper setup. Besides, you can virtually never get a data connection above 3-4K anyway so the 3G is useless airborne
There are several companies that make single leg options as iPad kneeboard solutions. Hands down the best one is made by MyGoFlight. Disclaimer: they are a Night Vision Pen reseller. But I have seen and tried many types and theirs is the best.
Apps: the two top apps out there right now are WingX Pro and ForeFlight Mobile HD. They each have their pusses and minuses. You can get 30 days free from each to see how you like them. Some people are willing to pay for both services to get the features they like from each app and switch back and forth as needed. I just stick with one and I prefer ForeFlight because I prefer their interface a bit better. As far as costs go they run about $75 per year for the standard service. They both allow you to see yourself on the actual approach plates, SIDs, and airfield diagrams, but that service costs more. If you want that it runs about $150 per year total: $75 for the basic service and $75 for the georeferenced plates.
Remember that the FAA does not allow any handheld devices to be certified for IFR. Therefore you are not legal to use this type of GPS, internal or external for the iPad, to actually fly approaches. But it sure is a sweet chart reader, specially when you can see yourself on it. And as previously noted above, some of the more advanced jets you may fly will have difficulty getting GPS signals in the cockpit.