One tutorial I can recommend is Howto: ISP-style Email Server with Debian-Etch and Postfix 2.3 Even though it’s for Debian and it does far more than you need but it explains how it all fits together. Sorry I cannot give you minimal configs, because you really have to understand what the options mean. So
After that edit /etc/postfix/main.cf according to your needs.
Also edit /etc/dovecot/dovecot.conf
wasn’t too far off.
No problem, as long as you can port forward through the firewall and make the external IP the MX for those domains.
I don’t have a preference on authentication as long as I can manage users from another workstation on the same network
No problem, passwd file is the easiest but you may not wish to give everybody an account so the MySQL method in the tutorial might be better.
I eventually want to install squirrelmail
No problem, OpenSUSE has that package and it will talk to dovecot.
I want to allow any wan connection to send mail to any address at my domain without authentication and anyone on the lan to send mail anywhere without authentication but disable any wan connections from sending mail to any other domains
Pretty much mandatory and the way the default setups work for a long time now, otherwise you will get exploited as an open relay for spam.
also, I want to configure a catch-all address for any users that do not exist already
Sure this is a good idea? You will get lots of probe spam.
Outlook/OE are going to be the primary programs accessing this IMAP and I want to require authentication for LAN or WAN connections
Dovecot can do authenticated IMAPS.
Again I recommend that tutorial.
If it all gets too much for you, there is a tutorial for a setup that dedicates a entire box and puts a Debian distro with a web managed mail server on it which unfortunately I cannot find right now. I think it’s on howtoforge.