I need to know if I should get a water cooler for overclocking an i5 4690k to about 4.3 ghz. Please help.
No. Unless you are very unlucky and get a bad chip, even an entry-level air cooler like the CM Hyper 212 EVO or Cryorig H7 should see you to 4.3 GHz and beyond.
Not unless you are willing to spend over $100 on it. I would go with the evo 212 for a budget or the cryorig c7 for more money. Also, you might want to go the Skylake route.