I just download all my music through Zune. Xbox Music sees it and I can play it through there, too. I try not to download anything I find using Xbox Music because it's invisible in my Zune collection if I do it that way. (i.e.: Zune sees Zune only. Xbox Music sees both Zune and Xbox Music. If you're going to use the Zune software, do all your downloads using that player - that way, your collection can be seen by BOTH)
I have a Zune HD and it still syncs fine using Zune software and Windows 8 Pro. Just download the same old Zune software (which you can still do at zune.net, btw) and you'll be fine. I have not tried using Xbox Music to sync with my Zune, but I highly doubt it would work anyway: The Zune software was built with Zune in mind, Windows 8 and Xbox Music were designed WITHOUT the Zune in mind.
I'll never get rid of my Zune software. I've got multiple copies of the install program just in case.