For such an old system, your DE matters more than anything. I’d recommend XFCE, or even LXQT. Definitely upgrade your RAM though, that’ll give it a nice boost - DDR3 should be pretty cheap now. Any cheap SSD should be a big improvement as well.
I’ve got Zorin installed on my mum’s PC (which is just as old as yours), and it works really well, even on a regular HDD.
I disagree, try running KDE on a 4th Gen i3 and compare it vs a modern system, the performance difference is pretty night-and-day. It’s not exactly unusable or anything, but it just doesn’t feel snappy and responsive, when compared to a lightweight DE on the same hardware.