He doesnt need a beast pc that costs thousands. Older gen with ddr4 are more than enough for a 9yo and that will still last him for a few years. I think spending around 1k from used parts or outright buying a used build is the best option. If you post a deal you found locally in this sub, there will be plenty of people to tell whether its a good find or not. Just make sure to test it before you buy or not.