2x750 corsair Cx750 or 2x750 CoolerMaster should be fine to run 6 x Rx 56 Vega cards. You will not wait that long though as the price of bitcoin is going down so the altcoins are recovering a bit, a rig of mine with 3xGTX 1060 3GB gpu is giving me 0.0007 btc/day today and it was giving me 0.0005 btc when the price of bitcoin was 7500 USD. So overall I think it is a good investment you are making and good luck to you.