7770k is not remotely best bank for buck.
If you are considering it, you dont need to get best bang for your buck, you are looking at a high end machine.
Forget "future proofing".
You should be looking to upgrade when technology advances and prices are attractive, selling your gear when you can minimise depreciation whilst replacing with newer tech when it makes sense for what you need/want.
If I was you id be inclined to see if you can pick up a second hand i7 6700k for a lot less than the Kaby lake new too.
Most of us are not going o0t be doing 4k gaming for a while yet and 1440p is the sweet spot.
You might consider spending a lot more cash on a monitor if you are a gamer.. that with a reasonable 4 core processor and your current card will give you a lot more for your $ than getting a new i7 and playing on a relatively poor quality 1440p monitor (if that is the case)
I have to say that 144hz at 1440p on a gsync monitor is a dream and it isnt my cpu thats being taxed ( I run 1080s in SLI)
You can ad another 1070 down the line or sell and upgrade.
If you do have a limited budget, in my experience second hand cpus , ram and graphics cards are all fine and perform for years. I wouldn't ordinarily buy a second hand motherboard or power supply. Very dubious paying a lot of second hand screens too. Obviously buying from sites like ebay where you can return for 30 days gives you an opportunity to test what you buy, relatively risk free.
I would have though for example, that you could get a 1080 for a new 1070 price (and an evga one too with the transferable manufacturers guarantee) a 6700k for $100-150 less than the new Kaby Lake.