I’m looking to install Openclaw on Old pc parts

I’ve got a few old PC parts (inc Intel Core i5-2500K and 16GB ram) and wondered how I could turn them into a working headless PC for installing openclaw. The only bits that I’m missing are a case and a power supply and I’m hoping to get those for under £20. I’ve looked on Facebook marketplace but nothing is available nearby. Which is weird because I’m in London and thought there would be much more results.