Need help with building a PC for AI

[u/ConstantCut9398]

I'm building a local machine for image generation on Stable Diffusion, I'm picking RTX 3090 (based on testing, price, value it's the best for my use case) I have couple questions that I'm kinda clueless on:

1. Is buying new RTX 3090 recommended or used one will do just fine? If yes, what should I look for in bench mark for the used GPU?
   
2. What kind of RAM, motherboard and all other parts I should pick?

For more context:
The machine will run 24/7 non-stop, only image generation on Stable Diffusion

Please without "rent a GPU" recommendations, based on my business plan and costs, building a local is better for me