On tautology: although it is unnecessary, it is a perfectly valid linguistic device. As I mentioned Virgil, I will use him again. In the Aeneid, there is a line in which he writes "lapsa cadunt folia". This means "dropping leaves fell". It serves to emphasize the motion of falling, and draws attention to a lot of contextual meaning. Unnecessary does not mean wrong.
I am aware of the origins of nota bene, and they illustrate my point. Having an em dash after it seems wrong, to me. Saying "note well--the cave is dangerous" looks more strange than "note well: the cave is dangerous", in my opinion.
Check the spelling of the first word in the quotation (: I believe this was written by another person because it broke from the style of the rest of the page, though.