This is the first guide I ever made, so it is not professional.

Most of the time we see kha'zix as an ad assasin for the first and most part, and tanky for the second part, however, I like to build him more as an ad utility assasin.
I use the cooldown reduction boots with furor, since he benefits extremely from cooldown reduction. The furor allows him to stick to his target better, and is proced on his primarily damage tools.
Ghostblafe gives the most flat armor penetration and a good active to chase your targets, as well as some cdr.
Muramana gives kha'zix his manapool to spam abilities. It's toggle is applied twice on his passive attack, and once on his taste their fear, which make him deal more damage even when his target isn't isolated.
The hextech gunblade. Some don't like it but if you look past the ability power it is a great item. It provides all the sustain kha'zix may ever need, and the passive of the item, which is reducing its active's cooldown, is also lowered when kha'zix auto attacks or uses his q on an enemy champion, making it potentialy spammable.
The last whisperer is just a must on ad assasins, ignoring allot of armor.
If your feeling rich an infinity edge quickly boosts your damage, together woth ghostblade you have 40% crit chance.