In the market for a new phone for Mrs Floyd. to get her to a currently supported version of Android? (She's using an S4)

Mainly calls and txt.  Very light user in the way off apps, Fitbit, some music & photos and potentially her City-Fitness gym-app to make session bookings.

Is she better off with a brand new Oppo AX7, a used S7 or something else entirely in the range of $250-$300?