Windows XP Pro, Outlook XP

I have a client that I do some computer training (Office stuff) that wants
to set up their network (Outlook) to be able to share calendars, etc.  I am
not a hardware person by any means, but I can see that they are not set up
to be using Microsoft Exchange (since the Delegates and/or Permissions TABs
are not there).

My question(s) is(are) this:

What is involved in trying to configure their system for Microsoft Exchange
(I know that is a broad question)?  They do have a server in their office,
but I have no idea how it is set up.  Is it easier to go ahead and buy other
software (like Office Calendar) to share this information, or is it better
to bite the bullet (or bullets) to get Microsoft Exchange set up.

I tried doing some research on the Internet to see what is involved in
setting up a network system to use Microsoft Exchange, but exhausted myself.
I did look some at "slipstick", which is where I ran across Office Calendar.

I'm just trying to find a low cost (or reasonable) means for a small
business (15 - 20 networked computers) to be able to share calendar/task
information.  They really only HAVE to have it on 3 - 4 of these computers,
unless the price is reasonable enough to do everyone.

Any assistance would be greatly appreciated!  TIA!