I'm not an expert, but have you checked to make sure that you have all of the repos enabled in /etc/apt/sources.list?
If not, if you could post the contents of that file so that we can make sure you have the right repos enabled?
Also, I'd suggest that you make sure you execute: "sudo apt-get update; sudo apt-get upgrade" and ensure that your system is up to date before you try to install KDE. NOTE: This will upgrade/update packages on your system.
Let us know how that goes, and if it doesn't work, we can look into next steps.