Only add new hardware if you can run your farm with money out of your own pocket for 6 month +
Since crypto is fluctuating, you dont want to be forced to sell to pay for that electricity and get rekt
Only buy EVGA card, best warranty /resellable value and transferable warranty, and maybe gigabyte too
Try to get GPU with backplate, easier to resell to gamer

I would not get 1070, try to find some 1080 ti for density and cost including all hardware per rig is pretty much the same
+Less card to sell and easier sell since it high tier GPU
5700xt seem promising too, but I would wait for the next card from AMD since it should be close to the Radeon VII
You can mine ETH to be safe but it low profit or you can also try to mine some speculative coin,
or 50/50 , Like mine enough ETH to pay electricity and speculate with the other rigs
Ive made way more mining speculative coin that if I had only mined ETH, like 5x more profit in 3 years on a lot of different coin to mitigate risk, coud have made way more too, but I like to hold for moon or nothing kind of strategy
Look around , theres still lot of really profitable coin out here,
Im currently making 3 $ day per rx 580
Get some ryzen, that way you can also do CPU mining, dont listen to those noob that say CPU mining dead
There some coin that so profitable to mine using CPU than you can ROI your cpu/board in less than a month easy
Just have to use your brain and find those promising coin