it'll get 144 fps easily but I would say it will reach around 200-300 fps. This is really overkill if you just play lol and I would recommend droping down to and i3 and 50 ti if this really is all you play.
LOL is not a demanding game on hardware. You could drop down cheaper components and get more or less the same performance. Afterall, most monitors max out at 60fps so shooting for 100fps has no meaning. Build for goal of 60fps.
If it's just for League a Pentium G3258 would work fine.
I would agree with this and maybe something like a r7260x or 750 ti for the gpu you can go even lower if you wanted but if you want to get he 144 refresh rate thats on the moniter these would be good.
I'd stick with the i5 in case you get interested in other games. If you know you'll only play LOL then I'd downgrade to i3 and 750ti instead of 280. The money you save could get you a 128GB SSD.