Do I need to use postfix or sendmail as my mail server?
Well, you could. But if the amount of work you need to set up the user (which, doesn't sound all that much to me) is what you're trying to escape, then neither of those packages will get you there. You still have to create users for both; either actual user accounts (with home directories and all that) where people can log in to check their mail or an IMAP mailbox. In both cases you're stuck doing a little bit of manual labor getting the user set up. Similar to what you have to do now. Creative scripting can help reduce that down to a single comannd that you need to run. There may be something out on SourceForge or the sites hosting those packages that's done this scripting already and it'd be worth taking a look there.
Hope this helps a bit.