Currently my desktop machine is an 12 year old Dualcore with a passmark rating of 1500. And though I'm starting to think of replacing it with more modern hardware before it collapses on it's own, it still chugs along reasonably well with Linux Mint 19.3 Xfce. With KiCad I can view and do small edits on boards like the OLinuxIno A64 (Project on github), which is a more sophisticated board then I will ever make myself. 4GiB or RAM also seems to be enough. KiCad needs about 800MiB when it's running, and having a web browser open at the same time needs another 1.5GiB.
Currently the AMD 3000G is among the cheapest budget processors for the consumer market and it has a passmark rating of 4800, so more then 3x the speed of my old dualcore.
Processors with this kind of speed cost around EUR50. Mobo's also start at around EUR50, and 4GiB of DDR4 RAM is around EUR25. So if you can scavenge the other parts you can build a x86 compatible for less then EUR150. Staying with mainstream processors for mainstream applications will probably save you a lot of headaches. but if you have a particular fetish for it then go ahead.
If budget allows the order In which I would consider upgrading:
EUR30 for an 120GB SSD, mostly for the OS, put data on a HDD.
EUR25 for an extra 4GiB of RAM.
EUR50 for a brand new decent and quiet power supply.
Monitor, keyboard, mouse etc, as needed. I've got a pile of those somewhere. Second hand monitors can be very cheap. I've picked up working TFT monitors from the dump for free. Watch EEVblog