Test your backups after you make them
it's a cheap and easy 99% solution
It's not a solution. It's a bare minimum requirement that doesn't solve for bitrot.
Honestly, I'd build two of these devices, one for local backups and I'd put one at a buddies house and do remote backups from your local device.
Oh what I'd do for usable upload bandwidth and reasonable data caps...
My understanding is that Storage Spaces is (as he says) MS's version of ZFS - does it not have the same data-checking features/ performance hit that 'regular' ZFS does?
No, it does not have the same data-checking features. Yes, it has a performance hit. Worst of both worlds. I've used it, and junked it as it was literally an order of magnitude slower than RAID5 via mdadm on Linux and didn't actually add any resiliency over RAID5 or flexibility as to grow an existing pool, you need to add multiple similarly sized drives since it doesn't rebalance. This is despite their marketing claims that you can add mismatched drives in an ad hoc fashion and have it "just work".
The only way to get Microsofts unproven resiliency benefits is to use ReFS in conjunction with mirroring (not parity) on the expensive server editions. Windows 8/8.1 does not support ReFS.
They also take 24 hours or more to credit accounts with electronic funds paid in
A luxury. Myki typically takes 48 hours when topped up online.
A fail-safe circuit will destroy others. -- Klipstein