My iPad is the 3G version. It has very accurate GPS. Wi-fi data is not available in the air, and the 3G connection is location and altitude dependent. I don't count on it, in fact I don't use it. GPS is good anywhere I've been.
There is no GPS connection to an external unit or an external antenna. You can get a 12 V charger cable; the one I got has a 12 V socket built-in on it for an additional device.
The chart apps I use (SkyCharts Pro is my primary one, with Foreflight as a back-up) cache the charts and the airport data. They'll store more than that but that's all I need. Because they are cached, they are available in flight and the correct one is automatically and seamlessly displayed, without user input. Position of the aircraft is routinely perfect.
You can make routes with multiple turnpoints or stops. You can display the charts in portrait or landscape mode, zoom in and out, etc. I've gotta say that it's pretty neat to be able to see the whole route at once and then to zoom in enough to read the small chart print. I think you can have the direction or travel as "up" but it's still a sectional chart and the text orientation doesn't change.
The iPad is heat-sensitive and can overheat in direct sunlight. When that happens it shuts down. You can restart it when it's cooled down. No damage, just the outage.
The iPad itself is very slippery. I bought an after-market "grip" that's a slip-cover for the sides and back. Works great except probably makes mine more likely to overheat.
I've had it to about 14,800 feet, no problems.
There are threads on VAF for the iPad and some of the apps. You can search for them. Sorry for blathering on, but I wanted to provide a summary of the thing as a pilot aid.