Your first question : Do not try to build the packages
yourself. That's what the .tar.bz2 files are meant for.
Instead use Synaptic or 'sudo apt-get install <kde-desktop>'
The packages are available :
.. set up the "Kubuntu Updates PPA repository" ...
QUOTE : "KDE 4.3.3 Available
Submitted on Wed, 2009-11-04"
Second question : yes, kdm or gdm will have a menu for
choosing KDE or Gnome.