There's a lot of value in using what you know, so if you know FreeBSD maybe that's your answer. Personally for OS I prefer Ubuntu or Debian. My mail stack is similar to your current setup - Postfix, Dovecot, SpamAssassin, and Roundcube for webmail. Ubuntu does a nice job of tying Postfix and Dovecot together for shared account lists out of the box. I'm actually using a PostgreSQL database for mine, but LDAP is simple too - you'd just define whatever LDAP search returns valid users/mailboxes.