There are many combinations, you can get an older PC with risers and a PCI splitter. Ram depends on the the OS but I would recommend 4gb at the min. You won’t need very much ram for your OS, if it will be only mining as the gpus will do the work.
As for the PSU you have 2 options.
Get a min of 1000 watt PSU make sure you can plug everything in also make sure that it will be compatible with MB.
Use existing PSU (if using a old PC) and then get a secondary PSU for your GPUs only
Thank you very much for your help. I appreciate it. I have a similar idea. I want to connect my motherboard (B450), CPU (cheap) etc to my old PSU and buy a server PSU (1200w HP) connected to all gpus. I will run my system on HiveOs. But also have another question, do I need to connect the two PSU to anything or can they work separately?