I'm looking into the tablet-thang as well, and I've heard positive feedback for Android/Nexus tablets, so you have a good choice at least.
As I'm still doing research for my tablet install, I can only answer one of your questions. In order to get around the 3G/4G issue, you would need to root your tablet so you can have Wifi-tethering. If you're unfamiliar with the concepts and benefits of rooting your device, look around on Google - you'll find plenty of information and "how-to's." If you have a phone that's capable of being jailbreaked (Apple) or rooted (Android), and you have Wifi-tethering set up on that, you can have your tablet connect to it as well. This way you'll always be online, which will create thousands of app possibilities, including Navigation.