Well, taking the electricity costs out of the equation, my first recommendation would be to buy a motherboard with the most amount of PCI-E available., I'd advise ASROCK H110 PRO BTC+
Then, take a look into which GPU's you will get and the amount of PSU's you will require so you can plan accordingly, I'd go for RX 570 or Nvidia 1060.

If you cant afford to buy all the rig at once dont worry, just buy the motherboard and 1 or 2 GPU's, as soon as you have more money start filling it.