I have a couple of old (N40L Generation 7) HP Microservers which I have used in various roles (FreeBSD+IPFW firewall, ESXi host, FreeNAS) and was fairly happy with. They are both off and distant to me at present, so I cannot check the specs. As far as I remember I expanded them both to to 8 GB memory and added a second NIC.
Good build quality.
Having drive screws and a screwdriver on the inside of the case door is a nice touch.
You can cram a 5th disk into the optical bay. I have not done this.
6 external USB 2.0 sockets.
Very fiddly to fit memory or an expansion card.
Getting an official BIOS upgrade may well require a HP support contract. This was enough to stop me from buying HP ever again. But see http://www.nathanielperez.us/blog/hp-proliant-n40l-bios-modification-guide
if you find that you really need to update your BIOS (enable hot swap, higher throughput for a 5th drive, lord knows what else).
No serial port - so you need a VGA monitor and USB keyboard for a console, or an IPMI card.
I read that the while HP say 2 a TB drive limit, they will work with bigger drives but not be able to boot from them, so I may well resurrect at least one of the N40Ls with 4*6 TB WD Red disks (with FreeNAS booting from internal USB, maybe with a second USB plugged in at the back).
There seems to be a £60 cash back running for the Generation 8 at the moment. Need to have an invoice dated prior to the end of January 2017.
As for cost, yes a NAS box can be expensive, even before you start stuffing big disks in to it. But how much is your data worth? I chose a FreeNAS Mini rather than building my own partially because it wasn't that much more expensive then building something with similar components.
Size is also an issue - 6 bay or more towers tend to be big, but you can at least stuff them under a desk or table. Rack mounts (like the interesting X-case one above) tend not to fit too well into a home. For some time I had a works HP DL360 sitting on a desk at home. Not too high, not to wide, but the depth
, and the noise
! I only powered it up when I needed to us it.