You have the screen and graphic card already? or no?
Running nVidia card on FreeSync monitor, you will not be able to use both GSync and FreeSync.
as for the games you currently play, I7 8700k would be useful but not much especially due to the extra price.
Going I7 8700k will make the system more future-proof but with certain gambling factor. For pure gaming rig and the games you play, I5 8600k has the better price/performance value.