I love Plex and use it on my NAS at home but you asked about using with a seedbox.

I use Seedboxes.cc they offer a Plex server, I do use this and it works well. Just copy the files to the relevant Plex directories on the seedbox then run the server interface in in your web browser to update. They have easy to follow instructions and if you do get stuck I have found their customer service to be excellent.