What are you going to use for the mail software? If you use Exchange, then the building of the Exchange server requires it to be on a DC, and installing it will make schems changes to the active directory for the entire domain, which is not reversable, so plan it out carefully first. Build the server, add it to the domain, then promote it to a DC role before attempting to install Exchange on it.
If you use something other than Exchange, I suspect you also will want to have the server already a DC so the mail has direct access to AD for user account authentication, etc. If you use a third party mail software, research all its requirements carefully first, especially understand exactly what it does to your Domain structure, Active Directory, etc., BEFORE YOU ATTEMPT TO INSTALL IT!
In any event, PLAN, PLAN, CHECK, PLAN, etc., BEFORE you make any changes to you Domain configuration or your can relly botch things, and if you do, you get to rebuild your Domain from scratch to recover from it, and I do mean from scratch, as you will lose all the Active Driectory data if you mess it up.
By the way, yes you can use one server for all that, but is this a amart thing to do (NO!) is the question. Putting your web site on your one and only DC is really dangerous, and exposes it to the Internet directly for all the hackers to play with. Put your Web site and the mail on different systems (READ: three different systems, WEB, Mail, and the main DC!), be sure they sit behind a good firewall, etc., and have the firewall direct the web and mail traffic to the correct servers to reduce your system exposure and danger as much as possible.
Before your mail will work you will need to tell your ISP to add the DNS entries to push the mail toward your firewall on the correct IP address also. Same goes for the Web site.
HTH,
David