If your dad is concerned about HDD power usage, would it make sense to use additional electronics to solve that problem? What about you keep your HTPC with RAID0, but configure the disks to spindown after x minutes of inactivity. That would mean it automatically spindowns the disks when you stop using it, and powers it on again with a 10 second delay when you want to start using it again.
If you use regular 7200rpm disks, who have idle power usage of 6-8W, and a 0.5-1W standby (spindown) usage, that would mean:
7W * 4 = 28W (awake-state)
0.75W * 4 = 3W (sleep-state)
I would assume this 3W is less than the power usage of an additional NAS. So i would assume you would want to talk to your dad about that.
What OS does your HTPC run?