I'll definitely agree on the Postfix and Courier IMAP front. I've been using both for quite a while, and haven't had any problems.

As far as which Linux distribution to go with, well, that's somewhat of a religious debate. I'd first recommend Gentoo, then debian, and definitely to stay away from the over-bloated distributions's, like RedHat.

There are some virus scanning programs that work with Postfix, like RAV, or Vexira MailArmor.