how do I view revisions to a question (presumably including tag changes)?
do I need a certain rep amount, if so how much?
No reputation is required, however, the link to view revisions only appears when the post has been edited in some way.
(because, what's the point of viewing revisions when there is only.. one?)
When there is more than 1 revision, simply click the date and time on the "edited by" section to view the revisions.
You can also enter the URL manually if you know the syntax, e.g.