Kha'zik Build - Which items are best? Questions on Lethality

Alright so with Dark Star Kha'zik out and how awesome the skin looks I want to play him more. I learned him well enough to get down his basics and I am fairly competent with him. There are probably things I could do better like clearing healthier and when to go into team fights, but for the most part I've done well enough that I can say I at least know the champion.

However I am not sure what to build on him after the warrior jungle item. I think Lethality is pretty good on him but how much and what works is sorta lost on me. From what I'm reading the Unique Passive of +15 Lethality doesn't stake with items, but multiple people are telling me they do.

So if I build Ghostblade, Duskblade, and Edge of Night would I be getting 45 Lethality or just 15? Cause from what I can tell you can only get Lethality from 1 item, and then some runes if I am not mistaken.

Additionally I'm not sure if I should add in any defensive options or if I should go all damage. I mean being an assassin I'd assume all damage would be good but I've seen people throw in one defensive item or another so you don't get blown up right away.

Anyways I've just tried black cleaver out. It was ok, not sure if I liked it, didn't really feel a big power spike, but then again maybe I just haven't had enough practice with the item.

So with Warriors and Boots as 2 of my 4 items, which should the rest be? I want to be able to deal a ton of damage on Kha and actually feel like I am an assassin. Sometimes it feels like even when ahead and isolating a target I don't do damage, or maybe I only think they are isolated.

Anyways many thanks for anyone who can help.

(Was using Khazik.GG for information on basic information, and LolKing at times, but I'm feeling either some of these sites im using are outdated or are taking much more general information or that of lower elo players)