To setup a UNIX PC as a postfix based mail server only (no webserver, login shell for users etc.), with no usernames (except the needed root user etc.) which packages would do best when working with postfix?

There is courier-imap, procmail, maildrop, vpopmail, sqwebmail and others mentioned packages that can do this, which ones are most appropriate? So many packages, so much confusion

The server will have several virtual domains to support, with each domain needing several email addresses, filtering, and autoresponders.
Users of each email address should be able to see mail either with POP3,IMAP, or web based interface, using some sort of securely sendable password

So the domains and email addresses for each domain will look something like this
onedomain.com has users jack,jill,john creating the need for email addresses jack@onedomain.com jill@onedomain.com john@onedomain.com

twodomain.net will have users bill,bush creating the need for email addresses bill@twodomain.net bush@twodomain.net

Which packages to use and are there any problems encountered when installing or using these packages with posfix?

I'd have to say here that you'd probably do best to use openBSD + qmail here. I get the impression that this is going to be a "toaster" system, ie one you're going to set up and then want to forget about; for such systems a high level of security is important since you won't be monitoring it closely enough to notice if/when someone breaks in.

As a side note, over half of the attacks I've logged and reported have been from dedicated name and mail servers which "were running fine so there was no reason to look at them" until I emailed pages of logs to their network administrators.