Ultra efficient mining rigs

I would love a little help on building some ultra efficient mining rigs. Need some hardware engineers on this with me. I myself am a hardware engineer but on the other hand my investor is wanting us to use refurbished motherboards and accessories but we can buy new RX480’s Looking to do a 400 x RX480 system in vegas. On solar power.

I have worked up some ideas. We have custom 1x upstream to 8 x 1x down stream pcie switches and I was thinking of using them with some ITX motherboards I’m pretty sure the key to a good motherboard is ensuring your northbridge and cpu have the ability to run 8 pcie lanes at the minimum (if you can disable onboard video) or 9pcie lanes if you can’t. I’ve found a few 12v nano boards with 1 mpcie connector we can use for our upstream you can disable onboard video and they have 8x pcie lanes on the northbridge and the cpu has 32 pcie lanes. My question would be or well what i need help on is finding a low cost solution this ITX board costs 480 dollars comes with a fanless cpu and no memory or ssd (comes with no 12v supply but we have tons of the HP 1200 watt 12v hot swaps with breakout boards and we can custom build all the connectors and wiring. In vegas it’s super hot so I think i have the cooling figured out. I buried 40,000 feet of 3/4" pex tubing down 10 feet underground so we can oil submerge all the video cards and cool the oil and not air condition the building at all. What i’m thinking is some laptop boards or hell even laptops we can use to stay at 50wtdp of the mobo with mem, flash usb 3.1 thumb drive and cpu. PM me if you can assist on makes models that are super cheap maybe couple years old