Normally, I wouldn't even begin to read a post that blabbers on the way you wrote it, but linux got my attention.
It sounds like the data being stored on the server is data that people do not want to lose. If that is correct, then RAID is your ideal choice.
RAID5 with mdadm is your cheapest solution. As for saving power by turning off idle drives, I know hdparm has that functionality, but I don't know how well that will turn out while using mdadm.