First and most glaring issue is the motherboard CPU combo. No point buying a basic motherboard and an unlocked CPU, especially since you want to overclock. You need a Z170 motherboard in order to do that. What's your overall budget? Are you concerned with appearance of the PC?

Assuming the budget is around what you originally listed, I threw in a Z series mobo, a very capable cooler (if not great looking) and tweaked a few other things.

PCPartPicker part list / Price breakdown by merchant

Alrighty, let's get to it

I'll definitely overclock the GPU and maybe the CPU

In order to overclock (OC) your CPU you need a motherboard that is capable and compatible with it. It's chipset should be Z170, the Z means it is able to overclock your CPU as well as some other features. You are getting an i7 6700 K which means you can overclock your CPU and it would largely be a waste not to

So I pretty much need suggestions on how to cool the whole system, especially the GPU.

First off, you need something to cool your CPU, the K series of CPUs don't come with a cooler, most people recommend something like the CRYORIG H7 for starters.

Your GPU is a little harder to explain, but I'll try my best. The Founder's Edition (or Reference) uses a blower style fan, it exhausts the air directly out the backside of the card and into the room, only problems are they're loud and still fairly hot.

The other type is open air it uses multiple fans to remove heat from the card and expel it into the case where the fans have to deal with the hot air. Generally, open air is cooler and quieter, but that is only when they have case airflow to effectively remove the heat.

The last option is an AIO (all in one, also a type of CPU cooler) it uses a radiator and liquid cooling to bring the heat to the radiator that is removed by the fans into the room, it's mostly unrecommended because it's more expensive and can leak (leaking is rare though) and takes up more space

What kind of GTX 1070 8GB do you guys recommend?

What I recommend From an excellent brand, pretty good quality, fits into the case

Last but not least, I have no idea about motherboards. Is the one I chose fitting? The only really important thing is WIFI and compatibility with the other parts.

Here is a more fitting one

"Graphics cards up to 315mm in length with front fans mounted (Cards wider than the dual slot bracket are not recommended)" The GTX 1070 Founders Edition is 345.44mm, but I dont know what "front fan mounted" means so I'm not sure, will it fit in or not?

The Founder's Edition is actually 267 mm, so it will fit with room to spare. The "front fan(s) mounted" is when you have chassis (case) fans installed in the front of your case as intake, since they're ~20mm in size the space for your GPU is lessened.

Onto your build

Overall with what I mentioned there's a couple things I would change as well

I don't know if you'll need more than 250 gigs of storage, you may want an HDD

EVGA G2 is better then the GS series