I use an ipad mini 4 (ipad 5 mini recently released), non cellular with FlightAWARE for GPS and adsb feeds. I also have a Garmin Go gps portable unit as a backup to the pilotaware.
I use skydemon but have used runwayhd. both very good, I prefer RunwayHD for the CAA chart overlay. Skydemon will submit a flightplan for you if required.
At work I use the ipad mini 5 in the machine which is considerably faster with good battery life but you could pickup a cheaper used ipad4 mini and have the battery replaced for less. You could also pickup an android tablet for considerably less brand new, skydemon has the same functionality on android as it does apple ios.
I use an ANKER power core 20100 for external power and a RAM Mount to stick the ipad on the screen, single sucker never fallen off. The anker has never run out of power on me yet, even on long flights, its a beast but quite portable.