You have a good recipe there. That's what I would go with as well. I have postfix, dovecot, spamassassin, clamav, amavisd-new, and postgrey setup and it works beautifully. I don't have as many users as yours but my server runs and handles the mail and blocks/tags spam very well. Postfix is one of the most friendliest MTA's out there to setup. I've never looked to see if there was a way to administer postfix over the web.
For webmail access, squirrelmail is very popular and not too complicated to setup.
I would probably go with SATA disks than IDE as read/write speeds would be faster and your company is only going to grow along with the number of mailboxes that you handle.