Biondo_ar,
Here is an example of what you want to purchase: http://www.newegg.com/Product/Product.aspx?Item=N82E16832116196 for Vista Home Basic and http://www.newegg.com/Product/Product.aspx?Item=N82E16832116056 for XP Home. This particular seller has a quantity discount.
My assumption is that you will not be building enough computers to make it worth the hassle of opening up a direct account with a Microsoft-approved distributor.
Please check with whomever is in charge of technology at the school(s) to which you plan to donate the computers. Most schools are most likely using XP, not Vista.
If the school installs a uniform installation of the operating system (most likely XP Pro) under an academic volume license, then the least expensive edition of Windows will satisfy the requirement for there to be an original license on the computer so it can qualify for the upgrade to the uniform installation. At the moment, the online retailers with whom I am most familiar are selling Vista Home Basic at a slightly lower price than XP Home.
If the school is not doing the above, then you would want to preinstall whatever version and edition of Windows that will fit in with what the school is doing right now. In such case, maybe what they would need would be XP Pro.
