I like NVR systems with PoE, Lorex makes some good ones and they do a decent job at trying to secure your device. Granted, that same security makes it more challenging to reset a password, but it’s good to know someone jacking your NVR wouldn’t be able to just use it.
For networks, much of it depends on budget and requirement. Do you want home security cameras on your network? Are you someone who works with large files all the time? Do you have gig plus Internet speeds? Do you own or rent? Lots of factors can affect choices. Start from there and build the infrastructure that works for your need. Don’t go looking to build a monster you’ll never really get moving, that’s a waste. Brand will become a factor once you look at the capabilities you need, you’ll see names come up over and over. At the consumer level, there are a few but price is also a factor. Not much help, I know.
I would learn about mtbf if I were you. Everything has a failure point (generally buried deep in product info), and when you start keeping it on 24/7, the hours will burn. Those fans on your GPU will give it up way before they normally would.
Finding a used server is not a good idea either. You aren’t in a data center and servers are super loud. Also, they chew up electricity like a hungry dog and his dinner.
As others have said, find a used desktop somewhere and a cheap KVM switch so you can use the same peripherals for both. It doesn’t need to be beefy by any measure (maybe drive space), just affordable.