First consider the following. Exchange 2003 is infinitely better than 2000 and fixes a lot of the flaws. However, MSES 2003 will require Win Server 2003 due to the ASP.NET functionality issues. If you are ready to go 2K3. That is the preferred solution.
Next hit the Exchange Web Site at microsoft.com and on MSExchange.Org. There are detailed step by step how to’s. Remember all the answers will not be in one document. Read and re-read, only then will you appreciate all the nuances.
Then experiment a set up in a lab environment. Depending on the number of users and the horsepower of your servers, you can choose an all in one DC and ES or a split up.
From your query, it is not clear what company set up you have. Whether you have an existing ES installation which you are expanding or setting up something new.
If all you want is POP3 and IMAP, I would strongly recommend a Linux based solution. May be even Novell’s server or a Sun low end a”all-in-one” system. Exchange makes sense where you will use the groupware, calendaring, etc. as features to compensate for the resource intensive nature of Exchange both in terms of computers and people.
With POP3 are you going to expose this to the Internet for your receipients ? How are you going to handle security ? If you want to use POP3/Secure, then there are two more layers you will need to add. SSL for encrypting the passwords and for that you will need to install Certificate Services. More resources on your computer front.
See http://support.microsoft.com/?scid=kb;en-us;262068&spid=1760&sid=312 for installation instructions. You can also find other Exchange 2000 info at http://support.microsoft.com/ph/1760.