It depends. Do you need it for gaming or for development.
It even depends which games you're playing.
But I think the i5-9600 would fit very well.
For the Motherboard you just need to find one with the right socket and right chipset.
https://pc-builds.com/calculator/Core_i5-9600/GeForce_GTX_1070/0Vk0UNlu/8/
I know, I know, this tools aren't always perfect, but I think in this case it makes sense.
The i9-9900K seems overkill for an 1070. There is no i9-990K