Running Stable Diffusion locally requires a high-end GPU for best performance. I'd recommend an Nvidia GPU with at least 8GB of VRAM, the more the better. Something like the RTX 3060Ti or 3070Ti comes to mind. You can build a PC around that.
CGDirector.com Parts List:
https://www.cgdirector.com/pc-builder/?=6NzRH
CPU: AMD Ryzen 7 5800X 3.8GHz 8-Core Processor ($238.00)
CPU Cooler: be quiet! Dark Rock Pro 4 ($114.19)
Motherboard: Gigabyte B550 Aorus Pro AC ATX AM4 ($189.99)
GPU: Nvidia RTX 3060 Ti 8GB - MSI Ventus 2X OC ($379.99)
Memory: 32GB (2 x 16GB) G.Skill Ripjaws V DDR4-3200 C16 ($86.99)
Storage PCIe-SSD: Samsung 980 PRO 1TB NVMe M.2 Solid State Drive ($99.99)
Power Supply: EVGA 650 B5, 80 Plus Bronze 650W Power Supply ($79.99)
Case: Fractal Design Focus G Mid Tower Case ($69.99)
Total: $1259.13
The above is a well-rounded build that would easily crunch through video editing tasks and the occasional Stable Diffusion. If you want to optimize it towards local AI Image creation even more, I'd save some money on the CPU (e.g. 5600X) and SSD (e.g. Samsung 860 QVO), and push the extra cash into the GPU for a higher-end model (3070Ti, 3080).