1. Hard to answer, because depending on the type of sites you are bringing in would dictate different server setups.
4. See #3
5. See #3 + message board and a toll free phone number
then you need more servers than just one, for sure. You're going to need lots of CPU, lots of ram, and if possible even SCSI drives. I run two fairly busy forums on one dedicated server right now, and they do a pretty decent job of eating up a lot of resources (running a pIII 1.2, 1gig RAM, etc..)
So just be prepared, if a few of your forum sites begin to take off, you will need new/bigger servers pretty quickly.