UPS batteries are generally good for 3-5 years, so if you get a used one then you may just be buying a nearly dead battery. A new, low-end UPS with a USB port will cost you somewhere between $60-100, which is worthwhile to protect your RPi and other servers.
I also have next-to-no Linux experience, which is why I wrote the beginner’s guide I linked to earlier. It was half to help others, and half to help me do it again in the future.
