What's wrong with just buying cheap older gen Intel chips? Still by far the best $$$/performance you can get.
Nothing really, they can be an awesome value.
My 2600k was, and still performs fantastic. The person I sold it too, absolutely LOVES it.
Why upgrade then?
I wanted to switch to Mini-ITX and gain USB C and M.2 compatibility. I also wanted better Hackintosh compatibility.
It didn't hurt that after selling the 2600k I essentially got the processor and ram for free, I just had to buy a motherboard.
If you do go older, I don't recommend going older than 2nd gen. and the Z77 chipset. Z68 chipsets work just fine, but the USB 3.0 system is an add-on chip so it's not bootable in many cases (something to keep in mind) and UEFI was also brand new on Z68, most companies were still trying to figure it out and some gave up before getting it fully implemented.
If you go that old, I also recommend I7 and not I5.
While single thread performance isn't any better, hyperthreading and multi-core processing has matured since then, so while at the time of release there wasn't a huge difference between a 2500k and 2600k, that difference has grown since then, meanwhile their resale values have gotten closer, making the I7 a better value than it was when new. And while it's true, most games are still terrible at multi-core, not all are, and as time goes on, they will continue to get better. You may be buying an older processor, but you are still buying it for future use.
Get a 2600K with a Z77 board and for the most part, the system is relatively modern by today's standards. It won't beat a new I7, but it still has more than has enough power for whatever you are likely to use it for. It may CPU bottleneck trying to game on multiple 4k screens, but if you can afford the video cards for that you can afford a newer processor as well.
Delid is still the most important aspect of buying intel 7700k..
And in my opinion the most moronic thing you can do with one.
"oh look I lowered my temps 1-2 degrees..."
And you risked killing a $300+ chip and voided the warranty, and for what? Wow, you gained a 0.5% increase on a benchmark. You risk a lot for very, very little in return.
If you are riding that overclock line to the extent that 1-2 degrees matter, you're pushing too hard and it WILL bite you at some point.
That said, it's your money.