If you get something like a qnap you'll be locked into whatever cpu / memory those things have.
I run FreeNAS for storage and Emby on my media server which is hooked to some Rokus.
My media server has an fx-8350 (upgrading), 16GB memory and a 1070. Once you start trans-coding stuff higher then 1080 you'll want a better cpu. Keep that in mind.
I'd recommend building your own NAS if you plan to use it for a media server / trans-coding. You'll have the option to upgrade down the line.