Facebook Marketplace is my go to here in the US. Pre built computers don’t hold their value for shit, so you can pick one up with a nice i5 or a medicore i7 office computer for almost nothing. Just look for Dell optiplexs or the HP/Lenovo equivalents. If you don’t need a lot of drive space (or are fine with external storage) those mini PCs litter marketplace and can go for stupid cheap.
I’d target 8th gen or newer, ideally 10th gen, but one that comes with an i7 might cost a bit more than $300. You can always go with an i3 for now, then if you need more power then the non k i7s tank in value after a few years of being out.