For the power supplies, I'd recommend getting 3 of these:
https://www.aliexpress.com/item/New-and-original-Gold-POWER-1800W-BTC-power-supply-for-R9-380-RX-470-RX480-6/32825827647.html?spm=a2g0s.9042311.0.0.xNt9YmIm running a mining farm with 48 of these PSUs for the past 8 months, and none of them have failed, you get ample power, and running 6 cards on each.
1x 570 consumes around 100W from the wall mining ethereum solo, and 140W from the wall dual mining.
Even thought those are average numbers, in any electrical components, there will almost always be that power surge that occurs for a split second; you do not want your PSU to blow up because of that. Thats why I'd always recommend anyone to calculate their power requirements to be at 80% of the PSUs capacity; so if you have a 1000W PSU, try to calculate that your components connected to the PSU to run at 800W, and leave that extra room for electrical surges.
18 cards will not run on Windows as of now, we are expecting an update in december that will allow windows to recognize 18 cards; where currently linux is the only way to go for more than 12 GPU. (Simplemining OS is a nice noob friendly linux OS for miners)
With the asus motherboard, you do not need the dual/triple PSU cables. You may hook 3 PSUs to the motherboard, and it does the job on its own.
The graphic cards are too expensive, even with canada's inflated prices.
A quick check on newegg.ca , I was able to build a cart of 18 RX580s, but different vendors, that can easily save you 500$ and If I were you, I'd configure each card's overclock/undervolt through their bios (time consuming, but worth the effort).
Prices are inflated as well in my country's local market (Dubai, UAE), but what I've done is I found the distributor's for these graphic cards in my country, and Im purchasing directly from them. In my case it was sapphire.
I would recommend getting a bigger harddrive, or at least a second cheap non-SSD drive for virtual memory. I had this issue before when I upgraded my rigs from 6gpus each to 12 gpu where claymore's cryptonote miner (cryptonight) requires to have at least the same amount of VRAM as virtual memory. In your case, it would be 18x 4GB which is 72GB + your OS which is around 20GB; a 120gb SSD would be a better option, and it isnt that much more expensive.