If there is mdnsresponder in stable, then install it. If there isn't, then you may use the one from unstable. 'Unstable' doesn't really nean your system will crash every ten minutes, but rather, there might be a few small bugs. You can add
http://packages.debian.org/stable/ (or /unstable/ or /testing/) to the sources list in aptitude or Synaptic. You can also lock or force a certain version to install.
KDE 3.4 is not in the Debian stable branch, so it's dependancies would not, unless needed by an earlier version or other programs. Either version 3.2 or 3.3 (not sure) is in stable. Why not install an earlier version from Synaptic or Aptitude? There aren't very many differences.
I don't know how it will affect KDE if those packages aren't there, but if it compiles sucessfully, and KDE can start up, you might be able to diagnose the problem (or ignore it) from there.