Hi John,
I got an iPad 1, 3G, 32 GB for my 50th birthday last year. This is the basis of the navigation system.
You can use a WIFI iPad as well, but it doesn't have a real built in GPS, and therefore you have to use an external GPS, of which there are a number, all with Pros and Cons. I wanted the 3G model, not for the fact that it connects to the cell network, but rather for the fact that it has a real GPS built in, that works well, even if you don't purchase a 3G data package. In terms of accuracy, GPS technology is constantly evolving and theres a couple of features the built in GPS doesn't have that are now a bit of a buzz, but compared to any other GPS receiver I've owned, the built in GPS in the iPad is far more accurate than anything I've used, and the GPS in the iPad 2 is supposed to be even better. So if you don't own an iPad, make sure you buy the 3G version, or you'll be spending an extra $60-$120 on a GPS receiver anyway.
As far as software, there are a large number of iPad navigation apps available, both simple and sophisticated, and mostly priced between 99¢ and $25. After trying the free trial versions of about 10 different ones, I've found GaiaGPS to be the most satisfactory, both in terms of its interface, and the responsiveness of the programmers to bug fixes and feature additions. Second to that would be MotionX HD, which has a much richer interface, but is far more dependent on cell or wifi service, and provides less in the way of real topographic data. These two apps, as well as a few others are being discussed frequently on Expedition Portal...
In terms of mount, RAM mounts out of Seattle have a bunch of iPad options. I chose this one:
http://www.ram-mount.com/NewProducts/Ap ... sivecradle
As you can see they have lots of variations. I use their 3" suction cup to mount it to the windshield. There is no reduction of signal with the Discovery heated windshield, or the Series truck aluminum top. I used this on a speed run of the Hunters Range Jeep Trail in my Series IIa, and Northwest Challenge in my Disco II and the suction cup held on tight, not dropping off once. Both of these runs were bouncy in the extreme, as my passenger can attest.
Lastly, I purchased an antiglare screen protector for the iPad. It gets quite dusty at times on the trail, especially in the Series truck, and while the iPad has no moving mechanical parts, I don't want to scratch the glass. The anti-glare really helped, as the screen is very glossy and the glass does reflect bright light. The anti-glare screen totally eliminated this while still maintaining nearly the full brightness it had before.
Oh, one more thing – power. You can run for about 6 hours off the battery, as the GPS app, since its alway on, drains the battery as quickly as watching a movie. I run an inverter built in both trucks, but there are a number of accessory plug chargers available for the iPad. I've found that when the iPad is doing it's thing with navigation, that the charger will maintain current battery state, but will not actually increase the charge of the battery. The other charge related thing is that sometimes you just don't need the GPS, say if you're driving a 30 mile gravel road with no turns. In that case, you can turn off the screen and save the energy, but the GPS maintains its lock and the program maintains all the waypoints, etc, so when you turn the screen back on, everything is right there in front of you.
Oh, and one more thing – other stuff. Don't forget that the iPad is darn close to a full fledged computer, so its easy to take your email, contacts, web access, a few books and movies, etc. along with you.
Most current programmers for iPad are also offering their programs for Android, so you don't absolutely have to have an iPad to enjoy the advantages of tablet navigation, however I think iPad is far ahead in terms of the user experience and smoothness of integrating software and hardware.