"....want to install kde-core only and start from there,
not the whole kubuntu-desktop-kde3 package. "
I don't think it is possible with the official packages,
all for /usr/lib , etc.
The "Pearson kde-3.5.10" installs to /usr/kde3.5/lib/, etc.
and thus kept separated from the KDE4 files.
Try downloading some of it, there is a 'patch' 90 MB,
which I guess contains the kde3.5 packages.
The usual definition for kde-core :
"...the bare-minimum required– of KDE. That is,
kdebase, kdelibs, arts and fontconfig."
It may be possible, may be not.