Total rookie with Linux ? Fine, remember to make backups!
You can go for a smaller Ubuntu installation like the mini iso [warning : this is current and not a release] or server install (2GB iso). Then after the installation open a terminal, or in the virtual console, type in :
sudo apt install kubuntu-desktop
The kubuntu-desktop is a so-called meta package which will install Kubuntu (In fact, you can have Xubuntu, Lubuntu, Kubuntu and Ubuntu all in one computer, and choose between them at the graphical login screen, but beware that the different screen saver software may conflict, and maybe there is more to be aware of).
After that is all installed, reboot and try it.