This is the way. Win is already activated, no need to pirate anything. Just get a >= 8gb flash drive, download the installer creator app from Microsoft, and reinstall Windows. It will activate based on the licensing level the machine came with, minus the crapware, which is from the manufacturer's OS image, not Microsoft's.
I’d pick up a NOS or tested used board on eBay. Make sure to tweak your search terms to exclude broken and parts systems, look specifically for ATX form factor boards from 99.5% and higher feedback sellers and you should be fine.
EBay has a ton of retired server hardware that’s in more than good enough condition for long term use by a home user. Non specialized form factor stuff is a smaller subset, but it’s out there in good quantities.
Also, although it would work great, the last thing I can imagine would be buying a new board for an old server CPU from Newegg, etc. The pricing for new server hardware, regardless of generation, is quite steep.
Definitely agree on the used idea. Ebay is full of used previous gen parts at great prices (stick with sellers with a deep history of 99% and higher feedback, avoid those with accounts less than a year old and/or single digit feedback. Avoid single digit sellers with suspiciously cheap prices for recent hardware like the plague - these are likely scammers.)
Personally, I’d avoid laptops if gaming is your primary interest. Performance does lag behind similar spec desktops, but more importantly, if something that isn’t ram or a storage drive breaks on a laptop, the whole machine is probably done. Not necessarily because the whole machine is unusable, but many if not most repair parts are model specific and can cost more than the laptop did.
Desktops can be repaired and upgraded per assembly, which makes them pretty kind to your wallet if gaming on a budget. I just scored an excellent condition 1080ti for around $150, and I know with absolute certainty that very robust off lease workstations from a few generations back can be had for $200-ish or less if you know what you’re looking for. Pair them with that 1080ti and you’ve got some decently capable hardware to play with!
I’ve got several gaming machines that I use to run everything from old stuff to heavily modded Skyrim VR and many new titles, and I pretty much only buy storage amd cables new. Everything else is purchased used on Ebay.