Obviously I am a little late to the conversation. But, maybe someone else will come along and benefit from my suggestion.

I don't like using my phone for my gps device. It just uses too much power. My little garming etrex 10 doesn't use that much power, and I can store a few extra batteries easier than a cell phone charger unit.

When I use my gps, it is with a paper map. I go to caltopo.com and print maps of the area with a UTM grid on it. Obviously I could use another grid system, but the UTM is so easy, and it is similar to my military training. All I use the gps for is confirming my location. I use my map a lot and generally know exactly where I am. But, sometimes I like to pull out the ol' gps and confirm. So, basically, all I use the gps for is to get that UTM grid coordinate. I don't use it with downloadable maps or anything else.

So, knowing how I use the gps shows you why I recomend the garmin etrex 10. $88 on amazon right now. You would be hard pressed to build your own for that price.

If you want to use the gps unit to store maps and look at maps on the unit, then a different unit might be better.