Do not buy her an Intel Macbook. They will be obsolete soon and she won't be able to get the latest updates for her Mac.
I recommend a base model Macbook Air M1. They're often on sale for 899 at Bestbuy and with the education discount at Apple you can get it for the same price anytime.
That old computer you're buying is probably going to need a new battery soon which 200 so by the time you get all that replaced you're looking at 500. Who knows how long the screen is going to last, or the ports, etc... Tell her parents to help spread out the cost if you weren't looking to spend more than a certain amount.
Apple is in a transitioning period where they're moving all of their computers to in house processors. Don't have your niece left behind outchea.