The easiest way to ensure no CP is posted is to disable hyperlinks and image posting.
That will of course be inconvenient to some users, but it is a small price to pay to not have to worry about it. It's also more simple than closing registration.
As far as the Dos attacks, I'll most likely be using a VPS with 1TB of transfer, so it would be very hard to take it down. For reference, 1TB would be enough to handle 1 million users a month. Since it will be hosted using one of my own VPS's, I'll be able to increase bandwidth at my discretion.
I think I'll be purchasing VBulletin 4 or 5 today, and I'll start constructing it soon.