Since I don't know what motherboard/CPU/PSU to buy, what would you recommend for the 4x3080 rig and 7x3070 rig (2 separate rigs)
Thanks
Your single most expensive item to buy after GPU is PSU.
If you cannot get your hands on one or more HP server PSUs, then read on.
Why build just one rig that in your case would require a monster PSU?
The price for a brand new EVGA 1600 Watt Gold rated PSU is through the roof; nearly $600 in my country.
It is far cheaper to buy 2 brand new EVGA 850 GQ gold-rated PSUs for the same total wattage.
So you could go the route of building more than one rig and total cost can still be lower.
The additional cost of buying one more mobo with CPU and RAM is small change in comparison, especially if you buy a mobo second hand.
You can get a 2nd/3rd hand ASUS mobo with a Celeron/Pentium/Core 2/i3 CPU for next to nothing. You can get one with cpu and 4Gig of DDR2/DDR3 RAM for next to nothing.