...I don't have any XP boxes but I'm thinking of putting together a low cost Linux Debian box as a learning experience.
People throw away perfectly good systems all the time, and those make *excellent* NAS boxes/servers. I know a guy who runs the electronics recycling at a local town, you wouldn't believe the systems that people toss out! In some cases he sees brand new systems with nothing more than a malware infestation, being dropped off as scrap. If you put feelers out to your friends and family, in many cases you'll find yourself awash in perfectly good systems suitable for redeployment rather than traditional recycling.
Also when raiding together hard drives is it necessary to have the same make and model of drives or is that really not than important?
Matching the drives is not important, but it would of course be best if they were all Enterprise/RAID class (which they are in your case). Scott.