My experience with Drobo is less stellar.
It seems with Drobo, I can throw any different size of drives in, and it just works. How can I build something like that on my own?
In the scenario above you could get 8TB protected by mirroring, or use a Z1, Z2 or Z3 scheme.
I've not invested in backup power, but this is one area that I'm a little skeptical of... nowadays all filesystems are supposedly advanced (log-structured records, etc) so in the event of power loss, in 'theory' you should only be losing at most a few minutes of work, depending upon caching. i.e. stuff that hasn't yet been written to disk might be toast, but everything else should be safe. Or can worse corruption still happen?
(quick edit to add: I'm not saying that backups are a waste of time, not at all, just that I'm questioning having a short battery power-supply backup)
Battery backup is usually req'd if you want to use write-caching on your RAID. You can also use flash backed RAID controllers, it's prolly better than a battery backup today.
Ultimately, keep things in perspective. The battery backup option might be the same cost as a decent UPS. You can also always build your own or get a decent zfs appliance.