Welcome to mining. I just put together my own rig this week, so I can’t claim deep expertise, but I can offer some opinions based on my own research.
CPU: Intel i3 Kaby Lake
You really don’t need CPU for anything in mining, so the absolute cheapest Celeron will be fine.
Ram: Corsaire Ballistix 8GB DDR4 2400
SSD: 120GB Sandisk 530MB/s
Same with RAM and HDD. You really only need maybe 2GB RAM (512MB on a good minimal linux distro), and I dunno, 12GB of HDD. Unfortunately the Z270-A mobo (which I also have) requires DDR4, so you’re stuck paying a lot for 2x4GB AFAICT. If you have an old hard drive somewhere, or even an >8gb usb stick, that should be fine.
PSU: EVGA 1000Wx2 Gold G3
You might also look into HP server PSUs + an ATX conversion board. A few of the speciality mining shops sell these, and they’re a lot cheaper for the same efficiency than regular PSUs. They require 220v input for full power though.
You won’t need a keyboard and mouse or monitor permanently if you set up your miner headlessly (straightforward on linux, apparently possible on windows), so I’d just borrow these from your regular computer during initial setup, then you can leave them unplugged.
8-Powered Riser Cables
Make sure to get the 4 cap risers, not the cheaper kind, especially as you’re running full power instead of underpowering the cards for power efficiency.
The Z270-A has a built-in power button, so you don’t strictly need this. Could still be nice to have on a cord though.
Microsoft Windows 10 Pro
This is of course a point of contention, but IMO if you have the free time to research, build, and maintain a mining rig, you should really consider learning linux sysadmin skills while you’re at it. My feeling is that a lot of the actual maintenance and tuning work that mining requires is basically the same as with traditional physical servers, and linux distros have a lot more tooling for running in this kind of environment than a consumer windows 10 OS. Plus, if the government outlaws crypto, you’re still left with a bunch of useful transferable skills along with your pile of GPUs.
disclosure: I maintain many linux servers in my day job.
From what I have learned in this forum, 1080ti or Radeon Pro Duo seem to be the best GPUs that offer the most Sols/s.
That matches my research. My overall perception was that nvidia cards are generally better at equihash mining, while AMD cards do better at ethash, though I can’t cite specific sources for that.