1. Sendmail is probably still the most widely used MTA but in my humble opinion it's awkward to configure and maintain. I prefer exim
while others swear by postfix or qmail. There are several other options but for the sake of finding information and getting community support these four are probably the best choices.
You should also take anti-spam tools (SpamAssassin, MailScanner?) and antivirus into consideration. Even if you and your clients don't use Windows you shouldn't be forwarding or even accepting infected email.
2. For a small office network an old 200MHz Pentium with 64MB RAM will be more than enough. Sending and receiving email isn't very CPU intensive, (I assume you're not using any graphical applications at all) but spam and virus detection can be. Basically it depends on the volume of mail traffic and what scanning you do, but like I said - sending and receiving mail in itself isn't tough on the hardware.
3. Perhaps here
. The Linux documentation project
has lots of good information. When you're starting to know in which direction to go or if you want to know more about certain types of setup, Google is your friend (as always). Oh, and don't forget Google Groups
for searching the Usenet archives. It comes in very handy, especially with this kind of things.