I had the same question. I tried to repurpose an old machine running Win7. I installed the Unraid OS and gave it a go but it was so old (Core2 Duo with 4GB RAM) that 50% of the CPU and half the memory was in use all the time, even when idle.
The first thing to understand is that you don't really "install" the Unraid OS since it sits on its own USB stick. At first I thought this was weird but then I realized that it works nicely because it doesn't really matter what's in the box.
My point here is that you should download the Unraid OS and try using your existing hardware. If it works then great but if you find it slow then upgrade the mobo, CPU, and memory. The Unraid OS will use the new hardware. Mobo is around $100, memory is about $150, and CPU is about $150.