what are the limitations of the microsoft "professional sample" of
exchange 5.5 and can CALS be added?

There are no "technical" limitations to using the Exchange 5.5
"professional sample" other than the built-in limitations of CAL
License use.  It is a fully functional copy of Exchange 5.5 that is
intended for examination and testing before a full license copy is
purchased.

As it stands now Exchange 5.5 and Exchange 5.0 mainstream support will
end December 31, 2003 but if you've purchased the Microsoft Extended
support then updates and service questions will extend to December 31,
2005.

Microsoft will continue to make "security release" updates to 5.5
until the end of the extended support dead-line of December 31, 2005.

Your only alternative option, and a better suggestion, would be to get
your hands on the Exchange 2000 download and test that out.  You will
need a Windows 2000 machine with AD (active directory) support to test
it but you ensure that if you choose to keep Exchange then you skip
the rather cumbersome "upgrade" from 5.5 to 2000.