How difficult is it to run your own email server?

[Mark Phillips]

I am thinking about setting up my own mail server (IMAP) so I can sync
email, contacts, calendars with my blackberry. I have been researching
email servers on Debian, and have a few questions. 

I need email for 5 users, 2-3 have Blackberries, and there are 2 domains
involved. On the desktop, they all use Evolution.

1. How difficult is it to maintain am email server? 
A couple of articles/forums recommend using a third party for email
since keeping an email server secure is hard and a takes lots of time.
Keeping on top of all the types of attacks and exploits people come up
with, and then install patches to prevent them from taking over your
server and using it for nefarious activities is a full time job. Is this
true?

2. What combinations of MTA, MDA, MUA's do people use?
I have run across several articles that combine Postfix, Courier or
Dovecot, and SquirrelMail or Qmail, MySQL, ClamAV, and SpamAssassin.
Which is better (i.e. easier to install & maintain) Courier or Dovecot?
What other combinations are better? How much time does it take to keep
all these pieces secure?

3. How much hardware do I need?
I plan on using the funambol server for syncing the blackberry, and that
requires a P4, 1.8 GHz, 200 MB disk space, and 512 MB of RAM. How much
more do I need for email? Could my email server fit on the same machine
or a separate machine? The funambol server is a J2EE application (tomcat
& mysql) (http://www.funambol.com/).

Thanks!

Mark