Received an old computer without the plug, need advice about PSU.

I recently received my boss's old personal computer, but the plug for it is lost. I want to upgrade the computer so I can play games on it, but I need a new plug first. According to what I have read from this subreddit, a quality PSU is one of the most important things to upgrade from a stock computer. How do I figure out what wattage of PSU to get without first buying a plug? Or is the plug a separate component from the PSU, so I shouldn't worry about this egg and chicken problem and should just buy any old plug?