I have to admit, while I've built, and worked with many RAID's in servers, I've never tried to use one in conjunction with AFP, NFS, iScsi or ZFS.
I have to wonder though if the OP is going to need to use any of the aforementioned in his home environment. Do you think any/all of the above will be needed in such a small network?
As to an OS dedicated to, and tuned for, data sharing. What's the learning curve on it? Is it easy to learn, understand, and use?
I'm not trying to be argumentative here. I'm just thinking, the guy doesn't want to spend $800 on an actual NAS device and wants to build his "NAS" himself at home. My suggestion is cost effective and will work perfectly in a small environment and for under $200 you can buy an excellent RAID controller card that you could run under any OS, be it windows, linux or UNIX.
I'm a firm believe in the KISS principle. I'm offering a solution which provides redundancy, reduces cost and also minimizes the amount of "new" information and/or technology the OP would need to learn to utilize said solution.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.