You can buy XP Home here on newegg for like 90 bucks. That makes you legal. Besides, you won't need Pro, unless you are running a domain at your home, and I highly doubt that. Pro only gives you extra networking tools and preinstalls the MS backup, but you can add backup right from the home CD. Otherwise it works just as well and just like Pro, I use it for my home systems.
OEM copies work just as well as retail and cost less. The difference is, MS won't support OEM, they have to supported by the OEM builder, which in this case is you.
Just be sure not to throw the CD Key on the outside of the wrapper away. Stick it on the case for the computer you are fixing.
Volume licenses from what I remember aren't nomally sold for less than like 25 systems or some such ****.
Technically you have to have one copy per machine, though I know home bodies fudge that line and do 2 to 1 and it will let them. But Newegg nor anyone here will support such a setup, nor recommend it. Including me.