First of all...How important is being able to play 4K to you? This will guide the other decisions.
To view 4K, you will need HDMI interfaces on your hardware that support HDCP2.2, and HDMI1.4a (30fps max) or HDMI2.0 (60fps max)
You will also need media player applications that support HEVC (aka x.265) for decoding 4K streams and similarly encoded video files.
If you build an HTPC, like ChuckChef, and you want 4K, you need a 4K video card. (= more $$).
You have several application (media player) options if you go the HTPC route (Kodi, Plex, Kodibuntu, OpenElec), and performance will not be an issue.
Or you can get a MyGica 4K (1800e)android box, and flash it to OpenElec
Flashing these boxes removes all the Android/Google OS and apps, replacing it with a lean Linux version, and converts the box into strictly a media centre = very good performance.
For everything but 4K, I took a cheap McGica520e, and flashed it using [HOW TO] KODI Linux on Geniatech Devices incl. Helix Builds
(Version 2 !! Do not use Ember version !).
There are other flashing options lately that work very well and are stable.
It is now a Kodi box and will play anything, even 40Mbps+ Blurays. Does not play HEVC encoded videos,...but that seems to just be the version of Kodi and probably could be updated.
I would also connect the NAS by Ethernet to your router..better performance and accessible by any device on your network.