Step zero: I compared different tablets and used borrowed from friends quite a lot before deciding for myself to hang my life on an iPad. Sustainable operations is vastly better then the uncertainty of android devices.
Step one: I recently changed from a first gen iPad mini to the Retina one for better readability in flight. I would definitely advice to go Retina and yes, the smallest 16GB version will do fine.
Step two: GPS. Your option 1 - don't use tethering in flight, you have to maintain two devices up and running = two time possible faults and what if aunt annie called for 45 minutes before T/O and the iPhone is empty? Your option 2 - same, you have to maintain two devices and connection must be functioning all time. Support for the external GPS vary from time to time and from update to update. I would not like to go that hazzle tango. So I advice to go 3 - versions with internal GPS. In that case you have a 100% standalone version as a backup plus you could connect to external GPS, in the unlikely event of reception problems. You could also take the solution whith you on other planes. Additional advice: slide a SIM card in, even when mobile data is disabled - the GPS fix does work better, if there is a mobile carrier connection, i.e. first fixes are very much faster with a SIM inside.