Kha'Zix core items: Hand of Justice + Edge of Night + Rabadon's Deathcap Remember Kha'Zix should evolve Executioner first, so he can crit with his ulty without Infinity Edge.
Kha'Zix is picky with items, he really wants Artifacts rather than normal items. If you really don't have artifacts, you can just go double Hand of Justice.
Evenshroud for armor penetration, Sunfire Cape for burn/wound.
Gargoyle Stoneplate is good since you are focusing on one 3 stars tank champion.
When to Roll/How to Roll
Roll at Lv7 mainly, since you are playing 3 costs reroll. This gives the best chance to 3 star them.
However, you need to be strong early game for the stacks of Rengar and Kha'Zix, so don't be too greedy of econ. Level up and roll if it's necessary.
You can roll a little bit even before Lv7 if you already have pairs. You don't have to win every round, just need to kill enough enemies for the stacks. But win-streaking gives more gold and preserve your HP. And you really don't need to go Lv8 and Lv9 for this comp. So again don't save too much gold.
Alternate version
When you have "Double Trouble" augment at 3-2, you can play an extremely OP variation.
in this varation, every Rengar will gain double the amount of Mana from Kha'Zix. Every Kha'Zix will gain double the amount of HP recovery from Rengar. And you also have stronger basic stats.
This version also shows us that, the main tank can be any other 3-cost 3-star tank. Such as the Krug (Riftbeast Brawler) we are showing in this builder, and potentially Rammus (Sprykin Defender) as well.
If there are a lot of Elderwood players in your lobby, it will be hard to 3-star Hecarim.
Scout the lobby to see which 3-cost tank champion is least contested. And also adjust it base on your shop, be flexible.