The 6300 certainly is overclockable. A different CPU cooler isn't necessary for running stock, but the regular cooler included with the CPUs from AMD are pretty bad and very loud lol
With the RAM, you want to make sure it has good timings as well, but generally yes, if faster RAM is the same price, might as well go with that lol
The 760k and 7850k are the same CPU, so you would see no performance difference, and it would be a huge waste of money. Definitely Definitely get the FX series over the Athlon series. General computing will be better, gaming will be better, browsing will be better, etc.