You find no ready off the shelf solutions that cost less than a few thousand. The fact you noted an old PC tells me you want to install something there.
I can not offer support for this here but will note that I've seen IT staffers use Linux and all the tools there. They usually spend a month to set it up. We don't allow that sort of work in our office since it turns out to be a support tar pit and black hole combined. I won't go near it because of the same reasons.
Now you know and you have choices to make.
-> If you want something easier, go look at NetLimiter.
We bought a home in a semi-rural area that's about .5 miles from the service boundary of the nearest terrestrial-based internet provider. Since we couldn't get land line service we opted for a satellite-based provider. We have the option of purchasing 10/15/or 25 gigabits per month at 12Mbps. After we hit our volume threshold they can throttle our connection back to best available after other customers have used what they need. We started at 10 and are now at 15 GB. Unfortunately, my family seems to blow through that monthly allotment in less than a week. So I'm trying to accomplish a number of goals on the cheap.
1) I want to monitor how much data we are sucking down and spitting up independent of their metrics, because I don't believe that we're using as much as they claim.
2) I want to limit what my family can download by IP and Protocol, perhaps even by site.
2a) I wouldn't mind setting quotas on the kids both in terms of time and overall data use.
3) We get unlimited 12 Gbps data usage from 12AM-5AM, so I'm trying to figure out how I can download and record streamed content as well as download other items on a scheduled basis.
I have an old desktop that I could set up as a multi-homed server, proxy, etc.