I use a iPadMini2 cellular version with internal GPS and 32gb. I think you will be ok with 16gb but when I bought mine I felt that I might need the extra at some point.
The most reliable source for charging, if you have your own aircraft of course, is to fit a Charge2 or Charge4 and you will have interference free reliable charging. All the other stuff for charging will give you grief.
http://www.charge4.co.uk/