I don't consider overclocking to be a good value. After you finish buying a more expensive motherboard (to allow overclocking), an aftermarket cooler, a slightly larger power supply, and possibly an unlocked "K" CPU, you might as well have just purchased an i7 instead of an i5. You're only looking at being able to get 10-15% more out of your CPU, and will probably spend around $100 getting that.
Regarding what is future-proof, that's hard to say, and it depends heavily on what you're doing. I expect an i3 6100 to deliver 30+ fps in most games for the foreseeable future, but one could well make the argument that it's better to just get an i7 now, because CPUs are replaced so infrequently, the extra $80-100 amortized over 5 years is a modest $16-20 per year, and up-front, and an extra $100 in a $1000 build is only 10% of your total cost.