@Taylor G. Hi. I'd start by just using what you have and give it ago. Don't spend any money unless you need to. And, if you watched the LTT video, even if you do need to upgrade for performance reasons, then a newer 2nd hand desktop will be fine. No need to build anything.
If it was me and I started from scratch, I'd look for a Dell Optiplex with a 8-10th gen processor, in a larger sized case. Plenty on Ebay in the UK for £80-100. You can add drives, network cards (2.5g) to upgrade as you go.
When you get your copy of HexOS, maybe come back to this forum for advice on how to try it on your current hardware, without destroying your existing setup. I'm sure someone will be glad to help.