We recommend a WiFi+Cellular iPad. These models have a built-in GPS chipset, were the 'WiFi Only' iPads do not include a GPS system at all. If you do purchase a 'WiFi Only' unit, you'll need to purchase an external GPS (usually Bluetooth) to provide in-flight moving-map capabilities.
NOTE: You do not require a data SIM to be installed for the GPS to function (although a data SIM is useful for updating weather forecasts etc when away from WiFi Access).
AvPlan itself will need ~3 GB of space if you're only downloading one single area (without Synthetic Vision). If you do not intend to download more than one area, nor store any music and/or video on it and just use your iPad as an EFB, then a 16GB device can be enough. To give you enough space so you don't have to worry too much, a 32GB minimum is preferred.
Choosing between iPad, iPad Mini or iPad 12" Pro is a personal decision. Most of the time, the choice of screen size comes down to ergonomics of the cockpit(s) you expect to fly in. Ensure that the physical size of your iPad works well in the space – the last thing you’d want is your tablet getting in the way of an important procedure/switch/dial/lever/control column.
Make sure you are also aware of the rules regarding EFB mounting in your country.
Minimum requirements are iPad 2, iOS 9.3.5
Australian pilots, please note:
In Australia, phones (even the new 'plus' models) are not considered a suitable backup device by CASA. You certainly can however use AvPlan EFB on a phone for convenient pre-flight preparation; flight planning, plan submission, weather/NOTAM self-briefing, etc.