I would strongly recommend that you get an external GPS device. I've had both garmin GNS2000 (which died after just over a year) and garminGlo which I still use. The accuracy is incredible compared to Apple's own GPS.
The "cellular" version isn't that useful for me, but then I have an iPhone and a MacBook Pro generally with me!
I fly with my old iPad mini on a knee board (which isn't great TBH) but with the exception of the battery life occasionally dying on me... (I now also carry at least 1 Anker portable charger - which recharges both iPad and GPS at the same time, tested it on an 8 hour day of flying and still had 80% battery on the iPad on landing)
With regards to space, I find that it depends on the number of charts you download. I regularly get asked to download 150MB of charts updates. My installation currently is taking up 1.1GB of space so 16GB (which really is only about 12GB after OS and formatting) still plenty enough if that is its sole purpose!
I hope this helps!
Alex