Quick update on the open-C buzz issue.....
I've now changed to Worth Brown (medium) strings and the buzz is still there. It's nowhere near as pronounced, but it's enough to annoy.
I'm not really sure what to do now. I think it must be an issue with the nut because the buzz goes away if the string is fretted. I'm really hesitant to do anything drastic. Maybe I'll save my pennies and take it to a luthier/tech some time.