You are forgetting the cost of cooling, K series do not come with a heatsink so factor that into your calculations.
I overclock mildly with my 6700K to 4.3 because I can, but honestly there's no real benefit. Previously with a 2500K I forgot to redo my overclock after a BIOS update and ran at stock speeds for 6 months without noticing, that's how much you need it for gaming. I consider overclocking to be something for enthusiasts and people squeezing performance out of a budget system, but it's totally not required for a good gaming experience.
Motherboard chipsets only really matter when it comes to looking at overclocking, I/O, and features if you are not overclocking and not using multiple graphics cards almost any board will do and honestly you will not notice the difference. The Gaming experience on a B250 board with an i5 7600 and something like a 1060 - 1070 will be near enough the same as a Z270, 7700K combo with the same graphics card. For the best all round experience you are better off spending the difference on an SSD than the more expensive parts.
On the other hand I ignore my own advice and buy the more expensive kit because I can afford it and I want it, comes down to budget really.