I'm about to install Plasma 5 on older hardware. What are the minimum system requirements of KDE Plasma 5?
If your computer can run KDE 4, you can run KDE Plasma 5 too. If your computer specification higher than http://en.wikipedia.org/wiki/Kubuntu#System_requirements, it's definitely that you can run KDE Plasma 5.
FWIW, from personal experience, I have run KDE 4 on a Core2Duo 1.8 GHz with 2 GB RAM and whatever graphics Dell built into their laptops almost ten years ago. As noted in another answer, KDE 5 will run on any system that could run KDE 4; in fact, KDE 5 has been slimmed down some from KDE 4, so requires slightly less performance and resources.