For a one-PC setup, the i7-5820k is a good option.
If you plan to go with a 2PC setup (including one of the 'cheap Xeon' builds that have become popular lately) then an i7-6700k is a better bet as it can still stream, just not as well, but will get better in-game performance if you end up offloading the streaming aspect anyway. (I got a 5820k and am kicking myself a little bit as the 2PC option is very nice if you have the money to spend).
Go with an i7. i5s run out of juice after 720@30, and even if you're nonpartnered and plan to stick at that res/rate to hit the 'golden point' settings, you can crank the encoder down to use much better compression (at a cost of higher CPU load).
I have an ASUS X99-A USB 3.1, and it's been MAJOR problems. I believe I got a defective one though, endless USB issues. But they have the version II coming out soon (or now?) which is supposed to have USB improvements. Up until this point, ASUS has been my go-to brand for solid performance... I'm waiting to replace the mobo to see if this was a one-off thing.
Plus if you go X99 for the 2011v3 socket, you can upgrade down the line to an i7-6950x for 10-core grunt, which is going to be the best single-CPU streaming build available at present. But carries the same cost.
Beyond that, good luck!
