I think unraid is great and I can expand up to 20 drives with little cost beyond the hdd's... Different size drives are not an issue. I would run mysql on the local machine, on a ssd. XBMC can be a little annoying to browse otherwise. I believe there are sab couch and sickbeard add-ons. Unraid worked out of the box with no tinkering for me, just had to build the pc and insert the hdd's then go through the setup.
I had a horrible experience with Freenas. There's nothing wrong with a nas it just seems overpriced and I can't really repair it, if it dies I RMA and wait. Upgrades are expensive also. A PC I just fix whatever broke. As far as reliability, not much beats a desktop. An UPS might be a good idea if you are really worried about this.
As far as power consumption you can build a PC to consume however much power you want.