Win2000 server migrating to SuSE Linux

spam assasin, shorewall.
the firewall spamware tools, they are part of the distro.

since there is no adware for linux, I haven't looked at anti-adware to protect windows machines.
( I have no windows machines )

the above 2 applications have kept me clean and virus free.
( on top of rejecting any email other than plain text )

spam assasin is my choice too but be careful on how you configure everything, it is possible to block out a lot of your good communications too.
Read this http://www.palomine.net/qmail/relaying.html

We just migrated from Exchange 5.5/NT 4.0 to SuSE SLES 9 and the Kerio mailserver. We opted to use the McAffee antivirus option with Kerio and it implements spamassassin as it's anti-spam tool.

Chose Kerio for their integration with Outlook and have been pretty happy with it so far. The data migration went well and it integrates with AD for authentication.

Your firewall is built in, (iptables).
Some scripting around it might help (shorewall,
or if you want a gui: fwbuilder).

For virus control there are: clamav, f-prot,
bitdefender.
To have you mail scanned for viruses use
spamassassin.
To integrate virus checking and spam checking I
suggest you look for amavisd-new, it does a
wonderfull job of finding the right virus
package itself (and it knows a few more than I
mentioned) and you can control it very well from
the config files.
It will also preforks a few waiting server
processes so it responds quite fast when
mail does arrive.

Kind regards,
Nico