What GPU are you planning to pair it with? The 6700k is still a great CPU if you can get it for a good price, which might be difficult considering the i5 8600k is likely cheaper and a touch more powerful in games. In terms of single core performance a core out of a 6700k would be just as powerful as a core out of a 9700k at the same clock speed, the two cpu's share pretty much the same architecture.
300 euros including the RAM might be a good deal. But I would price out a new i5 8600k system, if price is close I would personally opt for the 8600k.