i prefer to buy them from a store so that it comes with warranty like xenonpros.com or Global Premierre off Sugarloaf Parkway. They offer 1 year warranty and they are both legit companies with good products.

10k is what you like the best. Its really a personal preference. Based on xenonpros.com. The higher the "K" the less the brightness and the lower the higher the brightness. Then anything higher than 8000k will get more purple and less than 8000k will get more white