I agree with the others. At your stage of training, don't start using a GPS (any
GPS) just yet. Concentrate on the basics first, starting with dead reckoning, identifying visual features and such. Then master radio navigation, then master GPS. And by the time you start using GPS, don't try to do it on your own but get an instructor (or experienced pilot) to show you how to use GPS effectively, without becoming a slave to the magenta line.
As for your other questions: 3G is not required to operate SkyDemon. It's just that only the 3G iPads have a GPS built-in: The wifi-only iPads don't have a GPS. If you have a Wifi-only GPS you can get an external GPS like the Garmin Glo (but there are three or four others) and connect it to the iPad via Wifi or Bluetooth. These external GPSs are better, by the way, than the iPad internal GPS (higher sampling rate for starters), and they can be postioned in the best position for signal reception, so even with a 3G iPad some people choose to use an external GPS anyway.
At some point in time you do require internet connectivity though, to download SkyDemon itself, to download the airspace information (needs to be done every AIRAC cycle) and to download the latest weather/notams and such. Whether you do that via an internal 3G card, or tethered to your iPhone 3G plan, or via Wifi somewhere is up to you. It's not a SkyDemon specific issue.
I can testify to the overheating issue. It's a bit of a dilemma. If you leave the iPad on the coaming for best GPS reception, you run the risk over overheating. If you leave it on the seat next to you out of direct sunlight, you run the risk of signal loss. It's something you're just going to have to try out in your aircraft, as different aircraft have different characteristics in this respect.