Get the highest thread count you can. That will give it legs.

PCPartPicker part list / Price breakdown by merchant

Note: My prices never include Mail-In-Rebates unless specifically requested.

• CPU: This is a quad core cpu just like the Ryzen 3, only hyper-threaded.
• Cooler: The stock cooler will do just fine!
• MOBO: Pretty low end really. But you just won't need much. This should fulfill every basic requirement you could have.
• RAM: A single stick so you can easily upgrade later. 8GB should be plenty.
• SSD: Any cheap SSD will do. This size should give you enough room for now.
• GPU: The CPU has integrated graphics so you don't need to invest in a graphics card.
• Case: Mini-Itx cases are generally smaller, and they allow for smaller motherboards. This cases is larger, but cheaper, and micro atx motherboards are cheaper than mini itx motherboards as well. This case is okay (for the price), and it should give a blank canvas to your sis for modification!
• PSU: Lower end power supply, but semi-modular so it is easier to work with. Decent enough for the load you will be putting it under.

It is certainly possible to go cheaper than this. Cases can be had for $20, you can drop down to the Ryzen 3. You can get a cheaper motherboard by about $12 or so. Might require a bios update to support the Newer ryzen processors, but that is fine because AMD will loan you a single core processor for the task. You can go with a smaller SSD or HDD. Sufficient power supplies would cost around $30. I just happen to think the upgrades are very worth it.

PCPartPicker part list / Price breakdown by merchant

PCPartPicker part list / Price breakdown by merchant