I highly suggest the used market for the graphics card and motherboard. When it comes to mouse and keyboard, your personal taste is pretty important as well. I came up with this parts list, and you should add parts that you'd need:
PCPartPicker Part List: https://pcpartpicker.com/list/LMs3Qq
CPU: AMD Ryzen 5 3600 3.6 GHz 6-Core Processor ($167.00 @ Newegg)
Motherboard: ASRock B450M PRO4 Micro ATX AM4 Motherboard ($89.99 @ Newegg)
Memory: GeIL EVO POTENZA 16 GB (2 x 8 GB) DDR4-3200 CL16 Memory ($63.99 @ Newegg)
Storage: Silicon Power Ace A55 256 GB 2.5" Solid State Drive ($30.99 @ Amazon)
Case: Fractal Design Focus G ATX Mid Tower Case ($49.19 @ Amazon)
Power Supply: be quiet! Pure Power 11 CM 400 W 80+ Gold Certified Semi-modular ATX Power Supply ($67.06 @ Amazon)
Total: $468.22
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2020-05-31 10:43 EDT-0400
An excellent GPU choice would be an RX 580 or 480 from eBay, preferably 8GB, but if you actually manage to ses a ryzen 3 3100 or 3300X in stock, that is a great way to save money and put even more into the graphics card. You might even be able step up to a GTX 1070, or something like a GTX 1660 super.