I'd first decide on your GPU (Nvidia or AMD)

My builds all utilize Nvidia GPU's specifically the 1070 Ti's but depending on your location and market price of GPU's I'd say go with 1070's or 1080's.

Processor doesn't really matter too much, just make sure it's compatible with the socket size of the motherboard you go with.  High CPU power isn't what you need when mining, something like a Celeron G3900 will suffice.

Throw in 4GB of RAM minimum

As for the motherboard find one with PCI-E expansion slots for the # of GPU's you plan on running.  I use the MSI Z270 SLI PLUS on all my rigs with no issues (6 GPU's)

Don't forget a small capacity SSD and a set of PCI-E risers to go with it all

Finally, get yourself a PSU in the 1000 to 1200 Watt range that has either a Gold or Platinum energy rating

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