I am looking to replace a 10 year old file server with a NAS. I’d like to run Plex on it (for 10 users), and use it for storing security camera footage. I would also like to have redundant RAID capabilities in the event any of the drives fail (which is why I’m replacing my current file server as 2 of the drives have failed). I’d like to not have to replace the NAS for another 10 years.
If all you have is 720p and 1080p video collection then DS918+ will do a good job. But if you have 4K media then you should consider getting TVS-882 or TVS-1282 with i5 or i7 processor to be able to transcode those videos when accessing them remotely. If it is for jus local network streaming, DS918+ will play 4K videos just fine. If you need more bays DS1618+ will be an alternative for a local Plex streaming.
Synology also offers incredible surveillance software free of charge. That included mobiles as IP cameras, face recognition and linking with sales terminals.
The great thing about Sonos is that you don't need to pay for music subscription service. With the cheapest NAS at home like DS115j or any other, you can have your backups done for all of your devices and still use your network drive as a Sonos music library. Sonos will connect to your NAS via your network and let access your provided music library. If you have little more of IT knowledge, you can even set up your Alexa or Sonos with Alexa built in to play music on your voice commands ("Alexa! Ask DS Audio to Play Music!").
Nowadays it is so cheap to upgrade your old TV and make it smart. All you need is Google Chrome or EZcast alternative plugged into your HDMI port at the back of your TV. Now you have great functionality added to your TV to send a video file directly from your NAS to a TV. No need for wires and chargers and other mess. You can use your phone as a remote to go to your movies or family videos and cast that content on your TV during family or friend gatherings.
Amazon Fire TV
You may not have a smartphone, or you just simply prefer to control your content with a remote in that case Amazon Fire TV will be a great addon to get to your TV. It also plugs into your HDMI port and will play content from your NAS to a TV. No wires no mess.
nVidia Shield TV
It gets even better! If you want some more entertainment and all previously mentioned functions - nVidia TV is a great thing to get. You can use it for playing games and watching movies from your NAS. The greatest thing is - shield TV does video transcoding. So even if you have a cheap NAS with a weak CPU, in a combination of shield TV you can now automatically on the fly change video file size and format for a destination device. It is a great way to reduce network load and let older devices to play video content which normally was not supported on the device.