I think the Buy vs. Build question comes down to sweat equity.
Do you have the time to, or even better, enjoy doing a build. Selecting the parts, checking the compatibility, dealing with small problems and frustrations? Can you be your own customer support?
If you do you can, a build can be a quicker and more cost effective solution than that of an off-the-shelf solution.
But if you are just looking for a place to store your stuff, without having to worry or think about it, then a Buy proposition can be much more attractive.
You have four software choices really, for a DIY build:
1. FreeNAS, FreeBSD based turn key build solution - this can run ZFS, or Linux Filesystems (EXT3,EXT4)
2. NexentaStor/OpenIndiana - OpenSolaris Kernel, where ZFS came from, requires a bit more technical oomph.
3. OpenFiler - Redhat Branch turnkey, not as polished as FreeNAS but is has a Linux base.
4. Other Linux General Distro - Ubuntu, CentOS, etc
Within each of those choices you need to decide if you want RAID, and what favor and color RAID. Software vs Hardware, Raid-Z/Z2(sw) vs Raid-5,6(hw/sw) or lesser. If you go Hardware raid you'll need a Raid controller, if software you'll need a motherboard with enough Sata ports or a Sata Controller.
That leads to platform questions, how many potential drives? Intel or AMD? Low power or high performance? How many NIC ports?
I'd also make the decision as to Used vs. New, do you want to base your build around some eBayed components? Save Bucks with more risk?
Folks here ( as you can see from Hydaral ) tend to be very helpful to the DIYers, so you've got that covered.
