Ipad Mini2 would be the IOS minimum, the Nvidia on the Android side a hands down winner on that platform. Difficulty will be getting a Nvidia if you make that choice. I'm platform agnostic, have IOS and Android, Mac and Windows. Have used both with the platform. iPad Air 2 or Pro for capability, best screen but awkward. Samsung can be hit/miss for Android with the Tab S2 being minimum IMHO. Others "work" but they don't know what they're missing . There are arguments back and forth, but the DJI IOS platform appears to be the front of the development curve.
Software rumors are you'll need 2gb of memory for new incoming apps, should use that to set the floor. That puts you at Mini 4 because 2 & 3 only have 1gb
I'm an IT guy and I've thrown multiple things at it. I stopped when I hit an iPad Air 4 w /cellular. I lost heading display in Litchi because it only works on Android but I can live with that.