Well if your building a Linux box for a business I would go with RedHat, and the Pro version makes no difference. It just comes with more software on the CD, and perhaps the book or two for reading.
For you e-mail program I would either choose Postfix, or Qmail, as Sendmail is highly insecure. Postfix was intended to replace Sendmail. Qmail is touted as unbreakable, and there is a reward for anyone who can hack into it.