I have just started using KDE and would like to be able to switch between specific windows on my taskbar using Meta+1, Meta+2 etc. like in Unity and in Windows. Is this possible, and how?

I don't think it is possible. You can use window presentation (ala OSX expo) instead. You can also create shortcut to get to the specific window after pressing that shortcut (kinda cool feature).

You can assign a shortcut to every window (press Alt+F3 for the windows context menu), but I assume you want to use a pattern that fits the taskbar (ie. Meta+1 should activate the window for the taskbutton on the outer left) in which case you're on the wrong subboard; the shortcuts have to be claimed by the taskbar since its layout is private information.
(I've no idea whether and which taskbar plasmoid supports such, sorry)