I’ve used KDE on a Thinkpad T60, it’s about 17 years old, has 3GB of RAM, and a Core Duo. It ran surprisingly well. Replacing the HDD with a SSD can also make a noticeable difference, so you should consider that if you haven’t already. I also turned off a lot of the animations and effects for better performance.
I have a T480 that I’m very happy with. With shipping I paid a little under $250. It came with 16GB of RAM and a 256GB SSD (which I replaced with a larger one).