I'll give you my build and thoughts on itemization so far. Lifesteal isn't really a great idea on him, neither is Tri-Force from the playtesting I've done. I'll explain. In this champions current state he is a burst dps champion who looks to kill someone as fast as possible and disengage, waiting to re-enter to re-engage . Lifesteal means you plan on having a brawl fest with one, two or three people, in which case, you'll lose.
I tried Tri-force, I saw a few Kha'Zix (most actually) who think that is the way to go. I have to disagree, the sheen might be a one of but you don't need the slow (since you have a slow built into your q and you should never chase) and the attack speed is a meh on him.
With that being said, I'll give you two different builds and lvl up paths for him real quick.
Vrs non-rammus match
1 - Boots of speed, Pickaxe, Health Pot
2 - Boots of Mobility, B.F Sword
3 - Infinity Edge
4 - Brutalizer
5 - Youmuu's Ghostblade
Your item list should look like this at this point --
Boots of Mobility, Infinity Edge - Youmuu's Ghostblade
After that build accordingly, if they are building armor get a Last Whisper, if their ap is hurtin you, build into a Maw of Malmortius.
Priority on spells for this build R>Q>E>W, Evolve Q, Leap then Camo
Second Build (Rammus in game)
1 - Prospectors Blade, Boots of Speed, Health Pot
2 - Boots of Mobility
3 - Last Whisper
4 - Brutalizer
5 - Black Cleaver
Build accordingly after that, see above.
Spell Progression R>Q>W>E, Evolve Leap, Then Q and Lastly R
------end second build---
Now a few thoughts on Kha for Dominion. I take Boots of Mobility each time because you can jump away, stealth and then get away if you get ganked and I also Level my leap first when facing teams where one Rammus can come out, taunt you, turn on his built in thornmail and let his buddies own you, while you sit there.
Learn how to use his evolve leaps, try your best to not intiate with it if you can help it because you want to use it to get away, the only exception to this is if you are going to get the kill or assist and its evolved. You can leap over the side of the ramp down and your enemy will never know your behind them while they cap if they are on your two main points.
I hope this build helps, I've got over 700 dominion wins and its pretty much all I play.