I run Plex on essentially one of these (different case, but n5105/8gb ram, bought from AliExpress) and they're great little machines for it. Most of my library is 1080p, but I have run 2 simultaneous 4k transcodes before and it just keeps chugging along happily. I'm actually running proxmox on it, with Plex being just one container out of several, so it also has the grunt to do several simultainious streams and keep my mastodon server, torrent box, pihole, and a few other things running at the same time. In my experience, you'll run out of ram before ever chocking the CPU on a standard setup, so it might be worth upping to the n100 to get 16gs instead.
It wouldn't be any cheaper for them to ship without windows, because the windows os youre getting from aliexpress sellers selling budget pcs is almost always counterfeit :p but installing Linux on them is a breeze, they've got a full standard uefi BIOS, so just plug a USB stick with Ubuntu on it in, and install as usual.
Getting proxmox to pass the GPU through to containers is a little fiddly, but it got a lot easier since they moved to the 6.x kernel, and there's plenty of guides around. It could well be worth a look if you want to run multiple servers on one device
As for the GPU, they're unlikely to make a huge difference either way, but note that the n5105 has no hardware support for the AV1 codec, so any media you have or end up with in that format will need decoding on the CPU. The n100 igpu has hardware decode instead, so if you think you might end up with any av1 content, then that's the way to go.