Are the board and CPU BNIB?
I could give you OC advise but that's another thread/topic, otherwise this will get cluttered (by everyone) pretty quickly. If you want to OC your CPU there are loads of guides for Sandy, Ivy, Haswell and so on to Kaby/Coffee. Here's the guide for your CPU and I recommend following it as closely as you can, try to really take in the information even if it takes a few reads as Darkwizie's guides outline the fundamentals of OCing most CPUs with info relevant to this generation. Kaby=Coffee Lake, the only difference (in overclocking) is core count.
As for your current/future rig, if you got that good of a deal you really can't loose. 6 cores are still very capable and will be for some time and the board is plenty for you to upgrade to any CPU in the current line up or even an 8700k (which is easily delidded). The 9600k can also be dilidded if you can't get the OC you want but I'd try a better cooler first and see where you land. Get 16gb RAM minimum and check your board's QVL, pick something off that if you can. I realize that almost any kit of 3200Mhz ram will work on the board but I feel like if you stick to the QVL there could be some advantages when OCing the CPU or the ram. May not be true at all but I'd prefer the QVL myself, you can also look at a kit of ram and find what they have tested the ram on. I wouldn't want to pay for anything more than 3200Mhz ram and I wouldn't want less than 3000Mhz. Faster ram can show some improvement but you get the bulk of your performance gains at around 3200mhz. I'd buy that and try to OC it further after you get the core 100% stable.
For coolers I'd recommend the True Spirit 140 Power if your case will allow the height. The Cryorig H5 is a good option but it's not as effective. The H7 is cheaper but will also be a bit warmer. If you plan on upgrading to something like the 9900k the True Spirit would be a better option there, or you might want to spend a little more on an larger cooler. On a 6 core there will be little difference between the TS140P and a Noctua D15 (very large cooler) but there would be if you were using a 9900k. Personally I wouldn't want less than the TS140P and if you don't have a case yet I'd plan around that cooler height. Anything cheaper (that I've seen) also performs and it actually outperforms many coolers almost twice it's size and is almost on par with the highest tier of air coolers. The highest teir of air coolers can beat it by quite a bit but I've only seen it happen with chips that put out a lot of heat which makes perfect sense.
With the deal you got you can't really loose, you could use it for 2 years and likely still sell it for a profit. Or sell it BNIB for a profit now. Or keep it and use it forever. If you can get another deal like that LMK and I'll pay shipping