ASRock H110 pro BTC+ is the current best mining motherboard for rig density IMO. Very easy to get going out of the box. Since the slots are so close, you do need to cover the back of the riser x1 connectors with electrical tape to prevent them shorting out on the USB connector of the one next to it. Sapphire Nitro+ RX 580's are the best cards in cost/hash for Ethash and are excellent dual mining cards that run cool. 30+ MH/s ETH + 850+ MH/s DCR @ 130-140 W dual mining after a simple Polaris Bios Editor 'one click timing patch' Bios mod.
I've got 4 rigs running on the ASRock H110 Pro platform. It's a solid board but agree electric tape between the PCI risers is necessary.
All 4 of my rigs run 8 GPU's. 3 rigs are running the 8x 1060 3GB cards which produces 18-24 a day on Nicehash and the 1 is running 8 x 1080 Cards and that's anywhere from 32-48 a day.
Best ROI would be the 8 x 1060 3GB cards in my opinion plus the power consumption / heat is much lower. You should be able to build one of these under $3600 bucks...