That is an appropriate combo.
The i7-8700K is as good as it gets for gaming, it will run a much stronger graphics card if you want/need.
The psu you need is gated by the graphics card.
500w is recommended for a GTX1070.
http://www.realhardtechx.com/index_archivos/Page362.htm
I have no problem overprovisioning a PSU a bit. Say 20%.
It will allow for a stronger future graphics card upgrade.
It will run cooler, quieter, and more efficiently in the middle third of it's range.
A PSU will only use the wattage demanded of it, regardless of it's max capability.
I might look at 650w or so which will not cost much more.
Whatever you buy, do not buy a cheap psu.
Buy a tier 1 o2 2 unit from a list such as this:
http://www.tomshardware.com/forum/id-2547993/psu-tier-list.html