I want to buy a layaway copy of Windows XP Pro. I don't need to
install it right away, but I want to have one, just in case I get a
new computer and XP isn't available at that time anymore.

Obviously I want a legal copy.

As far as I can see when searching, I can find the following types:

Full version (about $114-128)
OEM version ($90-110)
Media only ($29)
License + media ($135)

So what's legal with what?

Since I sometimes build/upgrade my own computers, I believe I can
legally install the OEM version on such a modified computer, right?

But what is the media-only option? What does that not have that makes
it so cheap?

And what makes the License+media option so expensive?

Thanks for any clarification.