IMHO, I would highly suggest you start small, get your feet wet. 12 GPUs on one rig was what i EVOLVED to, but I would not suggest that as a start. What happens if your rig freezes? You aren't making money.
Get 2x 6GPU rigs, MUCH easier to manage. Even though you will have a few extra parts, I think it will be worth it in the end.
PSUs get an ATX PSU platinum 750w PSU and use it to run all your risers, mobo, cpu on it. Get a server PSU (or more)
https://www.deepinthemines.com/collections/power-supplies/products/94-platinum-1200-watt-power-supply-for-gpu-open-rig-mining-ethereum-zec-zcashhttps://www.parallelminer.com/product-category/power-supply-kit/Buy your Vegas, but test with 1-2 at a time and gauge how much power they draw while overclocking etc (this is an art in itself).
You will need a wattmeter. This is nice, because you will be able to tell how many GPUs that each PSU can handle. Consider that you dont want it running full load, it should have some head room.