First of all, two most important parts are MOBO and CPU, regardless the fact you can update and par nearly any GPU with a new build, however, CPU only fits certain socket.
Secondly, if the Intel CPU only supports memory clock up to 2666MHz why would you pair it with 3000MHz when the speed will be limited to 2666MHz anyway? Also, 2x4GB RAM works faster rather than 1 kit.
Third, the i8100 is crap compared to Ryzen 1500X it's not even targetting here, it's on par with 1400.
Lastly, if you really want to build Intel specs get an 8350K which is best budget CPU at the moment and Z370 which allows overclocking. Get a decent RAM and either save a few bucks for 1060 6GB or just forget about Intel if your budget is very limited, you can't make miracles with $750 especially when all parts are slightly overpriced in Europe compared to US stock/pcpartpicker.