Hello, this is my first guide so please don't leave rude or offensive comments, otherwise enjoy!
Kha'Zix is a very powerful, fast, bursting champion. While this guide does focus more on sustainability, with the runes being geared towards defense, he still will have almost no sustain in the first few levels (about 1-7).

Please refer to the chart above, in here i'll explain evolution's. Kha'Zix possesses the ability to 'evolve' his abilities with each rank of his ultimate. Each ability can only be evolved once and he can't evolve all four, except under certain conditions. There are two ways to get all four evolves. First, kill an enemy Rengar. It's easier than it sounds trust me, the hardest part is that you HAVE to kill him before he kills you, if he gets you first he receives bonuses instead of you. The other way is to get a pentakill. Rather challenging yes but a skilled played will get one. Now aside from that you will only be able to evolve 3 of your abilities. I evolve Taste their fear, Leap, and Void Assault, in that order. I take taste their fear for the large amount of bonus damage to isolated targets (12% of their missing health). I evolve leap for the massive range and that if you kill someone, its cooldown is refreshed, making you very agile. Finally i evolve his ultimate for increased mobility mainly. I do not recommend evolving Void spike at all. Mainly because the ability costs 100 mana per use and with this build, you don't have mana to burn like that. The bonus i can give it however is that if you get hit by the healing aura of it, it stacks meaning you caould get 3 times the health normal.