Helpdesk Statistics

I have been asked to supply a service wrap and SLA for a helpdesk serviceing 500 remote (dial in) users. As the service is not live, I need some statistics of the type and volumes of problems that the desk will field. I would expect a large percentage of calls to be password/access related, which can be delt with by a first line function. If any one has any statistical information whatsoever, I (and my client) would be very grateful