Control multiple MPD remotely

I need a way to control several Music Player Daemons remotely (from different networks). All of the MPDs would be using hardware Raspberry PI 3 and running PI MusicBox, Rune Audio or mopidy (depends on the freelancer suggestion). Each MPD would be on a different physical location inside a NAT; I need each MPD to be able to be controlled from a single interface (Im thinking maybe a VPN connection to a VPS that would have the interface to control all MPDs). Also there should be a simple way to upload files (audio files) to the raspberry PI so content can be uploaded. The objetive of the project is to have music boxes with stored music files in several places and be able to control them remotely