I haven't used an iPad, don't have one. However I have and use my iPhone, when my caching partner has my GPS. I suspect the iPad GPS is similar in operation to the one in the iPhone and as such it uses signals from cell towers and other sources to ID your location. As such it is virtually useless if it is not able to get any cell tower signals, it needs at least two and more is better. As long as you can see the sky the GPS will work, some of the new ones even work quite well under heavy tree cover.
Unless you have some very good reason for getting an iPad, I'd suggest you use some of that money for a good medium-priced GPS, some supplies and save the rest towards an upgrade if you decide you're serious about this crazy, wonderful past-time.