I use an appletv. I have the version with a Ethernet port. It plays everything I’ve thrown at it so far and don’t have to endure commercials. The downside is you have to create an Apple account to install apps on it and not all apps are available. It’s also expensive.
Try deluge, it’s been pretty bulletproof for me once configured.
Try using pcpartpicker.com. You can do your build on the site and it will list the best deals for the components you choose. As a bonus it will check to see if there are some glaring incompatibilities at the same time.
If the license is registered to that system changing the hard drive and reinstalling the SAME version of windows won’t be an issue. MS saves your system’s “fingerprint” and as long as you don’t change too many parts, it will recognize it and attach the license back to it.
If you do lose the license on reinstall, there are site that will sell you a cheap license, buyer beware, or ultimately you could switch over to something like truenas (core or scale) to run plex which is license free.
Sign up for the free trial and cancel it as soon as you have migrated your pst file.