If you are going to use the phone for business (I mean managing multiple accounts, follow-unfollow, photos uploads etc.) you'll need a powerful device to avoid lagging and freezes which means:
at least 2GB RAM (the more the better)
top CPU (Snapdragon 8XX, Samsung Exynos, Octacore Mediatech of the latest generations or top Intel Atoms)
Android 5.X
A 5+ inch screen will do nicely
Personally I'do go for ASUS Zenphone 2 - cheap and delivers top performance for your buck and a 5.7 screen. But any flaghship device will do. Avoid "average" devices - they will kill you with poor performance.