At least Plasma doesn't just silently crash for you - on my Ubuntu 15.04 system, it just dies and dumps me back at the login screen (and there doesn't seem to be any way to debug it, nothing in
Anyway, you can at least install the KDE 3.5 fork (trinitydesktop.org has packages) on Ubuntu 15.04, and I'm pleased to say it works pretty well. In fact, it is also significantly prettier (from the perspective of those of us who value clarity over smoothness, and are quite partial to icons that are distinct from one another, and to having nice sharp lines and contrasts to distinguish bits of the GUI from each other). The other option is to look at Mate (apt-get install mate-desktop), which is basically Gnome 2.x, continued (though they are doing the sensible thing and using the Gnome 3 project's libraries)
One good thing about Linux is that the Window Manager is distinct from the rest of the applications. So, you can mix and match window-manager, taskbar, start-button, panel, file-manager, applications. XFCE4 isn't a bad place to start (and parcellite is an alternative to klipper).