In my completely unprofesional opinion, and with absolutly no experiance in this area, I would advise you to skip freemium. I am guessing that if you go with freemium you are going to get a bunch of people using it who are just adding an FAQ to their freepages 'website' and are unable to or never plan to upgrade to a paid version. The money will probably be in going after big buisnesses, colleges, NPOs, etc. who want to find out what their customer's questions really are. (Or possibly you could have a freemium version that had advertisements in it, and have the ads pay for everything.)

I also recall seeing several people complain about how it is often those who are not paying for the service that cause the most customer service difficulties.