Often pilots will ask what iPad we suggest to be used with FlyQ EFB. Here's some suggestions.
Storage: iPad storage depends how you intend use it. Odds are you'll find more uses for it than 'just for flying'. FlyQ EFB will use about 20GB for all states and all items. If you normally fly between two states, then that's all you need to download which uses about 1GB. For a long cross country, download what you need for the trip and then delete it afterwards to free up space.
Cellular: You will get better performance and reliability with a external GPS or ADS-B device over the built-in. Still having the internal GPS as a backup is nice. Having data service activated is not required but its nice to be able to check weather or file a plan when free wi-fi isn't available.
Screen Size: Depends on your cockpit style. Some treat the iPad like a paper chart. They look at it, verify there position, then put it back. Some like to fly with it on their knee or mounted to the yoke.