Protect and Serve with Open Source server?

Wow this site is wonderful and hope someone responds to this post.

I will be replacing my existing 200~ Imail 7.15 Server with a newer server and wish to keep iMail but use a Open Source system to clean incoming mail of viruses and SPAM — I inherited this system and wish to make things right in once fell swoop if possible.
My question is which articles should I reference to implement the following.

1. Create a new MX record with a higher priority to “CLEANING” server that will spam filter and AV scrub incoming SMTP traffic (iMAC G3 400) – then route to the “REAL” new RAID 5 SCSI 35 GB PIII 500mHz with iMail.

I have read and began using your articles on Exim/SpamAss./ClamAV.

Do I need to use OS X Server or can I use OS X Desktop Jaguar?
I am considering accomplising this with Linux but as I see your hook is “Chaing one server at a time.” I wish to attempt this OS X.

Thanks in advance,

Metro305