if you want to build a new one
prefer to build a Nvidia rig.
go 1070 or 1070ti first. (you can choose 3 fan models for more cooling and silence)
if you want to build a ethereum rig with maximum performance/price ratio, you can find and collect Rx470 8 gpu's in second hand from gamers, (8g are very rare). Or you can find a second hand rig. they are best for ethereum. maximum performance/price lowest wattage/hashrate.
But my choice would be for the gtx1070 in second hand.
We are not in it for daily profit more the long term game.... but looking to make a small farm with about 10 rigs in total.
Thanks
Don't you understand what people are trying to tell you? If you believe in crypto and are looking long term anyway just buy the coins directly. Expanding mining at this stage makes no sense at all. Your electricity costs is way too high and gpu mining is dead. Sure you can mine for fun but you don't need to lose money investing in 6 rigs.
/edit: just did the math with your numbers. I got 28x rx580 8gb (well I don't run them anymore of course) producing around 860mh/s with 3800 watt power consumption, if I put in 15 cents for your costs roi would be minus 34% per day. Why would you pay for hardware in order to pay more for mining eth instead of buying it cheap. But if you are so keen on mining I can offer you a deal. You can have all my cards for free (all sapphire, run with low voltage modes) - just give me 1 eth back per month for the next 12 months ;-)