IO I expect that you either can or will be able to get Jep charts for europe. That was and is the position with flymap, though I am not certain whether they still support the Fly Angel I bought at some expense two years ago. That unit has quite long battery life and is also quite small. It straps to my leg in the Chipmunk, which is convenient when there is nowhere else to put it!
As a back up unit I would look for long battery life but ideally any unit should IMHO have the ability to download NavBox flight plans to it, which you cannot do with Flymap, Airbox or indeed the AV80R, which I was also looking at recently. All these need bespoke PC software, which is not as good as NavBox (well ceretainly the flymap one is not!) and means using two pieces of software to plan flights. This steers you towards something like the Garmin 495 or 295, which are a good bit bigger.