B250 boards don't allow CPU overclocking. Even if they did, they're aimed at lower end processors hence their budget price and budget build quality.
Again, you can't overclock on B250. Even if you somehow managed, you'd never achieve 5GHz, the board wouldn't be able to cope. 5GHz is also very hot as you have to crank up the voltage, something which a hyper 212 evo wouldn't be able to handle.
That is a good SSD, SanDisk are decent.
Waste of money, can't overclock on B250.
Games largely rely on single core performance. The difference between the same generation i5 and i7 is very small meaning it wouldn't be a worthy upgrade. Doesn't make sense to spend $100 more to maybe get a few percent in performance gains. i7s have hyper threading and are more suited to multi-threaded workloads hence why the i5 still has its place.
An i5 7600K is a very good performer single threaded wise, I can't see many other CPUs being able to outperform this to a reasonable extent. https://www.cpubenchmark.net/singleThread.html
Really isn't much point getting a 1080 if you stick with that monitor. A 1060 would suit that more, a 1070 would be decent. 60Hz will only display 60FPS, something which the 1080 at even ultra settings on most games will easily double. Overclocking it will also just be for pretty little numbers on your screen, your monitor can't display it and all you'll be doing is putting more stress on the GPU, heat wise for no apparent reason.