Well any computer with at least a pentium 3 can be a file server , the computer has little or nothing to do.
Why not just buy the four 1tb hard drives , and use them as four 1tb hard drives? This raid setup isn't really more reliable or faster then having just a backup copy of the hardrive on hand. It over complicates a very simple machine.
Once again almost any pc can be a file server. As for a case look into some antec cases that have the front drive bays kept cool by some 120mm fans like the antec 900. Or also look at basic server cases.
As far as power , hard drives don't use alot of power at all. If there's no gpu then almost any power supply over 350/400w would be fine.