If you want long-time software support on an Android device make sure to get one which is supported by AOSP-derived distributions like LineageOS. As soon as you get the device install LineageOS (or something similar) on the device and you're set (OTA updates and all - I get weekly OTA updates on a Samsung SIII-neo from 2014). Get a relatively popular device to increase the chance of the thing being supported for as long a time as possible.