You need to look primarily for three things:
- Number of SATA ports. With 3 you could set up a simple mirror plus a separate drive for system, but IMO 5 ports was better to set up a 4 drives ZFS raidz2 pool plus 1 separate system's drive. Having one additional port free for attaching another drive temporarily, e.g. for transferring larger amounts of data or additional redundant backups, is nice to have. The more SATA ports the better.
- Low power consumption. 'cause this thing will run 24/7 spending 50...100 bucks extra for a high quality, high efficiency PSU quickly pays off by saving you a lot of money on electricity in the mid term. It's quasi the drives and the CPU plus cooler that need to be powered. Estimate it, and chose the PSU a bit larger. Any HDD is app. 10..20W, SSDs at ~5..10W, and you are not going to play the newest 3D shooter with your 800W graphics adapter on it. So, you do not need >100W power reserve.
- Fast network adapter that is supported by FreeBSD (when in doubt, chose Intel); if the machine does not come with such, you get a replacement for a few bucks.
As the others say: Rest doesn't matter.
CPU doesn't matter much. Any up to twenty years old 64bit AMD/Intel will do.
8 GB RAM was quite enough, you don't need more than 16 GB.
GPU doesn't matter at all. Don't need it. All it needs is to provide some connector you can plug any monitor to it capable of using a text only TTY, and some USB or PS/2 port to attach a keyboard. And that's only temporary for to set it up or if something is badly messed up, since for everything else you access the machine via ssh.
My recommendation was to look for and buy some old, used machine for a couple of bucks, and check if replacing the PSU by a new and better one. Maybe adding a PCIe card to get more SATA ports, or a new network adapter. Plus a small, maybe also used drive for the system, if it not already comes with it.
Depending on what you get and perhaps need/want to replace/add that machine will come all together somewhere between 50...400 €. Hardly get anything new for that.
Of course the drives for your data pool come extra. But with any NAS those are always extra.
Those are the most expensive parts, depending on how many drives with what capacity, speed and quality you want.
I recommend rather buy better quality than larger capacity, because those are what the whole show is all about.
Buy new high quality drives for the pool, good quality PSU, and used old junk for the rest.
