Well, not cheap - decent. I'd not buy used MB, but no problem with used CPU.
It depends on your budget, if you can comfortably afford Z170 MB + i5-6600K - may be, depending on your monitor and graphics card.
the thing is that i5 is not powerful enough in certain combinations. For example GTX 1070 on 1080p@144Hz monitor would be ~30% not utilized with i5 in some games.
But if you are limited in budget, i'd putt those 100+ euros into GPU.
i7-3770s (it is boosting to 3.9GHz) + GTX 1060 will give you awesome experience on 1080p
if you have higher res like 1440p - GTX 1070 would be the GPU choice.