Ssreek
Guide
Tried plex with the same and it works almost flawlessly for 1080p videos. For 4k or hdr related, I feel it's better to have it on local storage instead.Just to add to what has already been said, it's best to start with the setup which is cheapest for you. This will help you better identify your usage patterns.
For instance, even if your old PC is a gas guzzler, it is not a bad idea to just get started with it. You'll get an understanding of the whole setup process and will be able to use it. Identify if there's something more you need. Once you know your usage patterns better, you will be better equipped to identify the right solution.
No point spending money unnecessarily when you already have spare PCs at home. Try the whole setup with those first.
The only issue is the spare pc needs to run 24x7 or it needs to be switched on everyday and logged in manually. As someone pointed out it adds to electricity bills as well.
2 other alternatives I felt okay were a good router with usb3 capability or raspberry pi with 4gb ram. I never used any of these, so not sure how fast it will be compared to a spare pc.
Will keep exploring.