If you're in the US, check out EPC's Online Store. They're an electronics recycler and they sell a ton of used electronics.
I've purchased three Lenovo micro PCs (now running my homelab Proxmox cluster) and a micro tower that is a dedicated Plex transcoder.
They have a ton of decent, 3 year old systems that would meet your needs:
- they're cheap
- most all come with windows
- Linux can more than likely be easily dual booted.
They even have some super beefy Xenon laptops.
Edit: this MSI laptop pretty much perfectly fits your reqs.