From your post I can understand that you still haven't bought any components.
Therefore I would recommend you to wait until end of May - Computex 2019, when AMD will launch new cpus with new motherboards that support PCIe 4.0
Not only new CPU's will be available but also the current ones will drop in price.
If I was building the system I would go for 3200 kit with c14 timings, looking at Crosshairs supported list, I would probably take this kit : F4-3200C14D-16GTZR
Then again it's not necessary and will add quite a bit to cost.
Overall I would recommend you that you take 2x8 GB now and then add another 2x8 GB later if you feel that capacity is lacking.