I agree, my 4690k runs fine at 3.9 GHz with the stock cooler. I'd recommend going the extra "mile" (~$20) and getting an unlocked CPU, especially if you have an AIO.
I believe it is. I plan on buying a larger, faster one soon, but my dad brought it from work and I have no problems with it right now.
When I first purchased my motherboard I didn't know much about building my own rig. But it does work well and performs good enough for me. And is Windows 10 that good? I've heard mixed reviews about it, but I can still upgrade right?