My current media collections & backup/storage solutions rest on a 3tb & 2tb Western Digital external hard-drives backed up with the rest of my system to BackBlaze. While they have been running perfectly fine, it is not an ideal solution for a media/pseudo-NAS installation.
Since my main desktop is currently running the Plex server, which means my whole desktop has to be on if I want to just stream one movie/TV Show. That is definitely not something I want to continue to do. I would be wasting a lot of power keeping my desktop on just to watch/stream media. My goal is to create a home NAS server that I can keep on and have all my media/backup/cloud storage on and access from across the world.
There are a few different things you need to consider when building a NAS machine as opposed to a regular desktop PC.
Power Efficient CPU - Chances are for a regular user, a home NAS will not need the newest Intel Core-i7 processor or a 12-core Xeon. You get probably get by with a 2-4 core AMD chip (normal processor or APU) or an Intel-i3. The processor should not be doing many CPU-intensive tasks and will most likely just be handling the files and serving out streams if you're using Plex. Since the machine is going to be always on, a power efficient CPU is very important.
Good case for drives and airflow - At least for my NAS, it is probably going to be put into a closet or corner and forgot about. You do not need any crazy case details or gadgets, just room for a bunch of drives and good airflow. I'm trying to find one with room for 4-6 2-4tb drives.
Drives made for storage/NAS systems - Of course, this system could run off of any type of hard-drive, but there are ones made for this specific purpose. You won't need SSD's for this unless you're insane, you can stick with normal 7200 or 5600rpm disks. From what I've read you don't even need to use the fastest disks as the data is spread out across different drives due to your RAID configuration.
You could use 4 5600rpm 4tb disks and still have great speed, as the main bottleneck might be other aspects of the build. The network will most likely have a 1gbps link and although it is very hard to saturate that, you don't want to skimp on available bandwidth for the SATA/Disk/Network throughput. You'll want to disks to easily be able to handle multiple streams of data.
IMHO, the drives will be the most important part as they will be doing the heavy nonstop lifting. Depending on your RAID setup, you need to be careful on the size & number of your drives. I will most likely stick with 4-6 of these drives, Western Digital 3tb Red's.
RAM - This one is much more important depending on the OS that you're going to use. Most NAS boxes only come with 1-4gb of ram, that is pretty normal for most users and that is all you'll need. Most consumer NASs just handle simple I/O operations and don't need lots of headroom. If you use an OS like FreeNAS that uses the ZFS operating system though, you'll need as much RAM as you can get, usually 1gb for every tb. This is due to the way ZFS implements redundancy and error/bit checking and caching and is extremely useful/needed for an OS ran on ZFS.
Network - This one is usually the bottleneck in most NAS installations. If you just have two users that save files to it every once in a while or watch a movie you should probably be able to get away with a 100mbps router/switch. Most home routes/switches are already 1gbps enabled so you shouldn’t have to worry about that. Network is important because the amount of data you can stream to/from your device will be dependent on the available bandwidth.
You wouldn't be able to sync up a whole music directory while watching a 1080p movie while transferring documents & while saving a presentation with a decent speed because of low bandwidth. It would be like trying to transfer 100 gallons of water a second down a pipe that is only rated for 45 gallons a second. As stated before, although it is very hard to fully saturate a 1gbps link, you want to leave more than enough headroom for multiple streams.
Once you put all of these factors into consideration, you should have a solid NAS system setup. Find a closet or corner of your house, plug it in and enjoy your never ending stream of entertainmet and backups!