Everything UnSpawn said is true. As you have told us nothing about what you plan to do with this, we can't do any better than give you shot in the dark guesses about what to run and how to run it.
I'll quickly answer your questions:
1) DNS - Bind, the industry standard
Web - apache2, again the industry standard
email - I suggest qmail, as configured by http://qmailrocks.org
All of those can be changed, and aren't the best for everything, but an awful lot of people run those services.
2) Assuming you mean a knoppix install and not running knoppix live off the CD, there will be no difference between it and ubuntu. I don't suggest you use either of those for servers, those are more "desktoppy" distros. You can install ubuntu without graphics, that makes an ok server. I suggest debian (stable) or slack for servers. No GUI, all CLI, live and learn.
Both knoppix and ubuntu are debian children, built off debian unstable. They are by and large debian. There is no functional difference between them. 1 is not any safer or better than another. Why did you think there is a difference?
3) All covered above.