One of my rigs is 13 cards, but I'm using the ASUS B250 Mining Export board... I think the processor is fine, if I remember I used an even weaker one on that.  The only thing I would be careful about is the power, the 1070 is rated at 150w max consumption, and for 12 cards that 1800w, so way above what that is going to support.  You're probably better off getting two smaller ones, and just adding the second one when you need the additional power.

On the risers, the only recommendation I would make is replacing the USB cables with quality ones you buy separate - with the risers I've worked with, 99% of the problems I have with them are the cheap USB cables they come with.

Good luck!