probably anything of the KDE 2.x era, 2.2.2 i think was the last one ... but i could be wrong there. You can find mirrors on that site you listed for all slackware downloads, and in there you should be able to find an older KDE.
ONE thing you will need to do is removepkg your current KDE...
the best place to do that from is /var/log/packages which stores all the packages installed atm.
do removepkg kde* and then the support files (as per your previous download).
As for doing all of this easily ... should be straight forward yes. but you might run into issues of getting the older program to work with the newer libraries.
You might be better of doing an install of an older slackware, and then updating selected packages ... though that WOULD be alot more work.