Disk space for plex server on Nvidia Shield

Hi
I plan to get a Nvidia Shield as my new Plex media server.
Currently i am using a qnap for that but it has very low encoding performance.
So what I want to do is to connect the Shield via smb with my NAS to get my media. Thus should nit be a problem.
But what is about the needed disk space on my shield for alle the features of plex. I want to sync media to my devices (offline) which need to be transcoded and stored. Where does shield store this? Can this be configuered to be stored on my nas or do I need to get the big version of Shield with 500 GB?

Thanks
Kwicks