Fortnite uses only 4 cores fully and extra 2-4 cores (partially) when multithreaded rendering option is used ( from fortnite settings ). So basically its one of the games where only single core performance matters so what everyone said about 9900k is good choice. You basically need fastest cpu:s around to get stable 240 fps in fortnite and it will anyway have some dips below 150-200 in endgame even with i9-9900k so you might need to overclock it for more single core performance.
I would however go for 24.5 inch monitor instead of 27 inch coming from 10 years semi competitive fps. If your going to invest that much money then going 240hz monitor is obvious, but i suggest going something like BenQ xl2546 or xl2540 which are 24.5 inch monitors, xl2546 has far superior blur reduction compared to g-sync or v-sync called dyac which even some CSGO pros use. XL2540 is somewhat cheaper but wont have dyac. Basically 50% of monitors used by csgo pros is either xl2540 or xl2546 and its a must on almost any event held atm. XL2546 is the only monitor which can strobe above 144hz meaning you can go even 180-240hz strobe
EDIT: you can also turn dyac off in xl2546 if you dont find benefit from it, however i found suitable settings in few hours for it using blur busters application. Im currently at 180hz strobing to reduce strobe crosstalk, however i use this setting only for csgo since i can never get stable 180hz in fortnite with my current rig before i update CPU and motherboard, fortnite i use 240hz without dyac and wont notice screen tearing even when dipping at 100-120fps
Colors look washed at first but there is alot of settings to tweak on this monitor.