1. 16gb will do Ozrunways, but you will have little room for other things. I have regretted only having 16gb - you accumulate lot of other stuff, like photo's, pdf's, music, etc.. 32 would be ok, 64 very comfortable.
2. I have also regretted wifi only iPad, as Ozrunways can get a lot of stuff that is handy with a cellular enabled iPad.
3. IPad Mini is excellent, and 10inch ipad is too big for a small "single" seat cockpit like a Tiger Moth or RV-8. A 10inch iPad should be ok for a more usual side by side cockpit. Mini is great for Ozrunways, and for general use, but recently I seem to be using the 10inch more at home in preference to Mini. Not sure why.
4. My experience is a 7inch mini 2 has a noticeable bit more battery time before run out than 10inch iPad Air 2 with just wifi selected.