I use Discourse as well. Tons of flexibility and the head developer is a Ghost user himself.
It checks all the things on your list:
Has email capabilities to notify when a comment is posted
Can also serve as a sort of mailing list by itself since users can receive digests of new posts
Anti-spam plug-in available (and works well)
However many moderators you want
Plus more:
You can set pattern matching/regex so if you didn’t want your comments to go to a single category (like Blog Comments), and you happened to have other forum categories that corresponded with your Ghost tags, you could make it so that each individual new Ghost blog post comment has its own corresponding post in the appropriate Discourse category.
You’re already familiar with Discourse. You’re on it right now!