Looking suggestions for my upgrade

[defblue]

I have a I5 3570, and I don't have a very expensive budget to upgrade to this gen's current tech. I want to re-use most of my hardware, including the RAMS which are DDR3, and I was taking a look at an I7 4770/K or an I7 4790/K, so I can only change motherboard and CPU and just re-use all the rest, do you guys know better hardware or another platform that supports DDR3?
Also, I'm not from the US so US prices are far from my reality, looking for used parts is my way to go right now.

 

[The prime mediocre]

An i7-3770K would be a good drop-in upgrade. I don't know where you are or what the used market is like there, but in the US it's pretty affordable relative to current i7s.

If you're going to go ahead and replace the CPU and motherboard, you could go two ways: an LGA 1150 board + i7-47xxK like you mentioned, or an LGA 2011v1 X79 board and an Ivy Bridge-E CPU with more cores (i7-4930K). The latter would obviously be more expensive, but probably not close to the price of a Coffee Lake i7.

Technically, the best mainstream DDR3-compatible CPU is the i7-5775C, one of the short-lived 14nm Broadwell desktop processors. But they're fairly rare and thus expensive.

So... going off US eBay prices, an i7-4770K + Z97 motherboard would run $250-300, roughly 40% less than an 8700K + Z370 board, never mind the cost of DDR4. An i7-4930K (6c/12T @ 3.4GHz) and X79 board would be about $350, and get you fairly close to 8700K performance but you'd have to shop for motherboards very carefully--they seem to retain a lot of their value. And, again, I don't know where you are or if any of this is useful to you. But the relative prices might be similar.

Hope this helps, good luck.