Is this a good build?

I'd honestly go with a 8600k CPU if it's just for gaming. It's not only a bit cheaper, it outperforms the 1700x in pretty much every bench I've seen, and some games by a considerable margin. I also looked at a 8600k vs 1700 (non X) bench, and the 1700 struggled to maintain good CPU usage, even though the test bed had high speed 3200 RAM, which is best for Ryzen because it's Infinity Fabric is tied to RAM speed. The CPU usage was literally consistently half or less of the 8600k.

https://www.amazon.com/dp/B0759FKH8K/?tag=pcpapi-20

http://www.guru3d.com/articles_pages/intel_core_i5_8600k_processor_review,1.html

Now granted, you can get a 2x8 kit of 3200 speed G. Skill Ripjaws on Newegg right now on sale for 3 more days for $165, but with Intel, it's not as necessary. Though for only $9 more, I'd get it anyway honestly.

https://www.newegg.com/Product/Product.aspx?Item=N82E16820231941

You can actually get a decent Z370 MB cheaper than the one in your list too.
https://www.newegg.com/Product/Product.aspx?Item=N82E16813144113

So for, $2 less even with the higher speed RAM, you'd get a bit better CPU and RAM performance. Also, check before buying from Newegg because they used to not charge sales tax in my state, but now they do, and it's 10%.