1440p is the sweet spot. Very affordable these days to hit high FPS at 1440 including the monitors you need to drive it.
1080@120 is definitely low budget tier at this point.
Check out the PC Builder YouTube channel. Guy is great at talking gaming PC builds, prices, performance.
For the 40mm impact munitions in the list. Riot control gear.
“Grenade launcher” here isn’t very specific. Could be like an M203 or something, which is a tube launcher that lobs 40mm rounds up to about 100m. You can absolutely get 40mm frag grenades and blow shit up, that’s what most people think of when they think of a “grenade launcher,” but that’s not what it’s on the list here. You can lob gas canisters, smoke, illumination flares, etc etc etc, just depends on what you’re trying to do.