If this is purely a gaming focused PC build then sell the 1800X and go for the 8700K...The 8700K is a stellar gaming CPU that will be just 1 to 2% of the 9900K in gaming performance and more importantly it is not as absurdly priced as the 9900K. the 8700K also overclocks extremely well to 5GHz in most cases.
If money is no issue and you just want the outright best, then the 9900K is the CPU to get.
If you do go the 8700K route, then get a Z390 motherboard and not the Z370 as you will at least be able to upgrade in the future to a 9900K if required.
Finally the 1800X will also game very well especially if you are gaming at 4K where the GPU becomes more important but if you are into high refresh rate gaming or simply want the fastest gaming CPU for the money, then the 8700K will be the better bet.
Hope this helps..