Kha'zix build? Ok, well it looks like you're trying to build him as a straight AD carry, which he isn't. He's an assassin through and through, so you want enough of a damage spike to explode an AD carry, a mage, or a squishy support, and then enough survivability to be able to get into a team, do that, and then get out alive. Sustained damage (so your general AD carry build) on him isn't that great because he's melee and squishy.
First item is Ravenous Hydra, almost no question about it at the moment. Lets you clear waves with no effort which is great since void spikes chew through mana if you use it for wave clear (and even better allows you to isolate your lane opponent easier), and simply adds a ton of damage and lifesteal.
Next thing you want is armour penetration, so that means either a Brutalizer (into a black cleaver or ghostblade later), or a Last Whisper. If there's a lot of armour on the enemy team, that means both, but if you only take one take LW.
Boots-wise, depends. Lucidity(?) (the cooldown ones) or Mercury treads are your best options, depending on how much crowd control you're against. Tabi can be good if you're against a lot of AD. I used to like Mobility boots on him before they got nerfed as it gave you complete map control once you had wings if you were even slightly ahead.
If you want more offense (really the items above should be enough to destroy just about anything you want to jump on) then a Bloodthister is probably the choice. Manamune is also an option but takes a long time to build up. I'd only recommend it if you're playing top lane and in a matchup where you have no choice but to play really defensive in the lane, as it tanks your offense for a while in exchange for mana/health (from void spikes) regen.
Generally after that you then want defense. Guardian Angel is nice for a last item since you jump in deep a lot so you're likely to die. Randiun's Omen is awesome on him actually. Lot of health, armour and a slow for good measure. Banshee's Veil is great too for the spell shield, or a Warmog's if you don't need that and the MR (after the Randiun's). On that note full tank Kha'Zix is something I'm messing around with and it's absolutely hilarious to play.