My two cents:
1. GPS can be a valuable tool, but it is no substitute for a paper map, compass and a pencil. Always carry these in addition to a GPS unit.
2. The navigation app provided by your cell phone carrier may be useless without a cell signal, but your cell phone's GPS capability is independent of a phone signal. There are several computer navigation programs and apps that work with tablets and laptops, both independently and in conjunction with your phone regardless of whether you have cell service. Learning about these is beyond the scope of this forum. You might start following the two GPS-related subforums at ExpeditionPortal.com:
http://forum.expeditionportal.com/forums/45-GPS-and-Computers-(Hardware)
http://forum.expeditionportal.com/forums/46-Mapping-Navigation-GIS-(Software)
3. Simple is often better. You can spend big bucks on a unit that fits into your dash, but what if you want to use your GPS to take a hike to that perfect fishing spot that is off limits to motorized vehicles? That's why I like hand held units.
4. Learn how to use GPS before you buy anything. There are classes. I first learned as a volunteer adult leader at a Boy Scout camp. I later took a class offered by REI.
5. Borrow or rent a GPS unit before you buy. There is no better way to find out what you need and what you don't.
6. I am partial to Garmin units because it is the dominant company in the marketplace and therefore there are numerous tutorials, maps, discussion groups, etc. for them. Lowrance and Magellan are also good brands.
I have been using a Garmin GPSMAP60CSsx handheld GPS unit in my offroad vehicles for many years. The screen may be smaller than an in-dash GPS or an app for a Tablet, but I have no problem reading it. It is a rugged unit, and although no longer available one can sometimes find them for sale. The newest versions of the same series start with the Garmin GPSMAP 64.
GPSMAP60CSx
GPSMAP 64
Have fun.