16 Gig is plenty for aircraft use
The lowest end model is plenty for using WingX or ForeFlight or SkyCharts. With each of them full US maps take between 2 and 4 gig.
If you are going to use it for music and video then you might want more. I have an iPad 1 16 gig, and have 2 movies, 30 e-books, and about 100 games for the kids, SkyCharts, ForeFlight, and WingX and have a couple gigs left over.
If you don't want an external GPS then you have to purchase one of the 3G/4G models, as they are the only ones that have a GPS built in.
If you don't need the latest and greatest, you can now get a Apple referbished iPad 2 for $349 or $469 for the 3G one. The factory referbished ones come with a new case, glass, and battery, and have the full warentee. (So basically new).