Ann has a teenager and she wants a desktop. She wants to keep it down to around $400, though. Leo says that eliminates anything from Apple, as his first recommendation would have been an iMac. Ann should understand that at that price point, she won't be getting quality components.
Leo advises going to Dell. They have a good selection of budget desktops. But remember, she'll get what she pays for. She should consider refurbished computers, but she should only get them from the manufacturer. Leo found this Inspiron One 2020, which is an all-in-one PC that looks to be adequate for their needs, and is only $469.