That's the bare minimum, yes. Limited subset of styleable content / tags would be better though.
Waaay to complicated. Bear in mind were talking about a message board with a high amount of activity, defined character limit per post, which structurally would have to have style tags or even rudimentary css stored within the post information. That's going to be a database headache calling all of that 10's of thousand times over and over. Alt tags will just give you the text you would have written anyway but can either display it as a picture or text. That was you don't need to link to a text only version of your OT, you're not giving a complicate set of tools, even if only at a basic level, that more than likely will be a formatting and rendering disaster putting it in the users hands.