We have a really old Nexus 7 (made by Asus) and it's a dog these days, can't get updated to the newest operating system, etc. It was decent in its day, though. And that might not be the Nexus 7 you read about, since I think they re-use names sometimes.
Our newer tablets are both Samsung Galaxy Tab whatever-was-current-that-week. Both
work very well, both are challenged for viewing in direct sunlight. Daylight is OK, as long as it's at least a little shaded.
I've not met (or shopped on) an Android tablet that doesn't have GPS. I've also not come across any Android nav apps that need
Wifi for anything other than occasional chart updates. Do need Wifi for news feeds and so forth -- and "current weather" apps -- but most of the other apps we use for other purposes altogether different also
work Wifi-less.)
For
navigation, I'd say the larger the screen, the better... so you can get an idea of detail and context and the same time. With smaller screens, you end up sacrificing one or the other too much... or fiddling with the zoom in/out functions a lot more.
I've found it useful to find out what new tablets Otterbox make a protective cover system for... then go shop on those tablets.
I haven't emphasized
storage. Mine is 32Mb model, wifey's is 16. We do have to pay attention on hers, but then we also put in a micro-SD card... so paying attention is usually just taking care to store big stuff (e.g.e, charts) on the card.
-Chris