I have this one:
Amazon.com: Synology DiskStation 8-Bay (Diskless) Network Attached Storage - Black (DS1812+): Computers Accessories
It might be a bit overkill, but I like the fact that I can buy up to two 5-bay expansions for it as well, so if one day I'm worrying about my 4K rips taking up too much space, I can easily expand without issues. Synology also has a great "operating system" which has all kinds of neat shit that I wasn't aware of until recently.
For example, I have a lot of free time at work, but I can't do really fun stuff like jerk off, watch tv, play video games, read books, etc. I can fuck around on my computer though. So I transferred my entire jumbled mess of music downloads from torrents and newsgroups onto an external drive and plugged it into my work computer. With the Synology Cloud Station, I assigned a particular directory on my work computer to synchronize with the Music directory on my NAS at home. Now any time I organize another album (you know, renaming it properly, getting album art, etc.) and put it into the directory on my computer at work, it automatically uploads and synchronizes it at home.
I could do the same thing with any directories if I really wanted to. It is like having an unlimited sized Dropbox for no monthly fee.
On top of that, with the music app on the NAS, I am able to connect to it with the Synology app on my android phone, and anywhere I have a connection I can stream my entire collection to my phone.
It also has a PLEX app, but the processor in mine isn't enough to transcode HD movies. I let my HTPC (an Intel NUC that runs 24/7) do that, but if you bought a higher powered version you could even have it run PLEX all the time just from the NAS, and who cares if your computer is on?
It is expensive, but my Synology NAS is probably one of the best things I have ever bought, without question.