This build is for a good damage early game and better survivability for the late game, focusing on damage with your Q to isolated targets. For evolutions of Kha'Zix, I suggest evolving your Q first for damage, your R second for the damage reduction, and your E last for moblilty and easier target separation. pretty simple but avoid trying to jump in on a group of enemies, you won't deal nearly as much damage as you would if you tried to find an enemy that is solo.

as I said before these runes are for durability, because without any added health or armor. Kha'Zix is just a big squishy cricket.

go down the offensive tree if you feel like you want to be the high damage assassin, but remember, unless you get the drop on your target(s) you'll probably get bursted down pretty fast because you're probably the biggest threat.

go down the defensive tree if you want to avoid dying the second you get caught.

Pretty simple, you can go high damage all game or you can opt for some durability late game.

R>Q>W>E

you're going to max you Q first because it is you best damage tool
W second because of the heal and ranged damage
put 1 point in your E early for the utility and mobility

you're going to want to evolve your Q first, R second, and E last

(you can really swap the evolution of your R and E if you want to, but the passive your R gives you is 40% damage reduction)

your gank path should be:

wolves, Blue buff, wraiths, small golems, Red buff, wraiths again, wolves, then check your lanes and see if any enemies are pushed up too far and gank.

if your team is winning all the lanes just continue to farm the wraiths, wolves and small golems. keeping an eye out for good gank opportunities.