I have a 9700k and it did wonders for my FPS in Arma. Used to get like 12-15 FPS (during large official events) with a shitty AMD processor and jumped to 40-50 after the upgrade. Like previously stated, an 8700k would also improve your performance. Honestly, liquid cooling isn't really necessary if you don't plan on overclocking the CPU. Yes, stock fans for CPU coolers aren't the greatest, however they are rated to provide enough cooling for the CPU out of the box.
When I purchased my 9700k, it didn't even come with a cooler. I did however overclock mine to 4.9 GHz, so I went with a Corsair liquid cooler. Basically it comes down to your budget and whether or not you intend to OC it or not.