Thanks for the feedback guys. So I finally went with these configuration:

> 8x MSI GTX 1080ti Gaming X
> 8GB RAM
> Biostar TB-250 BTC Pro
> Intel G4400 Skylake Dual-Core
> 8x Molex risers
> WD Green 120GB SSD

To power this, I bought:
2x CoolerMaster V1200 Platinum Power Supply

Will the two V1200 do the job fine or should I get an additional small power supply to separately power up my motherboard and risers?
And how much hashrate should I expect?