Yeah a lot of people will say to just use your old PC thats lying around and it is a great way to start and learn but if you want something with a lower footprint (low power, quiet, cool) you're better off buying something more suited to that task. Laptops or mini PCs are much more suited to that.
Id love to have a server rack one day but I just cant justify it drawing so much power when I can live with the drawbacks of a smaller server.