People misunderstand Kha'Zix.
He's the best mid offtank champ there is.
His Q is nothing more than some extra manamune farming and something to proc sheen/trinity.
For evolution.
His W more than triples the damage output, because it's also adding his passive into it.
His E gives him great mobility. Plus the resets on kills/assists is absolutely great. You get either another AoE nuke, or an escape from a daring dive and kill.
His R gives him that 40% damage reduction, and another second of 40% move speed. The stealth part of it isn't that great, especially if you're on their turret or they have oracles. But the fact you get that extra mobility and tankiness makes it a great choice.
His Q is really only good for taking out a single isolated target. They can't even be near minions. Sure it adds a hell of a lot of damage to those isolated targets, but the opportunity just doesn't present itself enough. Any good team with good positioning wont let you get that extra Q damage in. If they do, it's probably their tankier player, or the other 4 are dead.
His W is the complete opposite of that. It does MORE damage if they group together. Because you're hitting them with 3 AoE nukes.
The 50 extra range isn't necessary. You're slowing them with your W/passive and leaping massive distances, and speeding up by 40% for 3 seconds.
Why mid? Because the moment you have tear of the godess, cooldown shoes, and level 6 with lv 3 W and W evolved, you can push the lane right to their turret and go hop on some quick easy ganks, and be back fast enough to push it again.
My skilling order:
W E W Q W R W E W E R E E Q Q R Q Q
Evolution: W E R
This maximizes your sustain, push, farm, and overall harassment by maxing out your W first.
The E leveling gives you that much needed cooldown reduction.
The Q is just good for last hitting minions and farming your mana, because of it's low mana cost and short cooldown. In laning, I would rather auto attack to proc my passive over using Q on a champion.
For items, I like to rush manamune and cooldown boots, then right after that try and get frozen heart. With the right masteries, this will sit you at 39% cooldown reduction, allowing you to spam the forget out of your W. The mana from manamune and frozen heart should be enough to keep you farming and harassing for a long time. Even better if your jungler lets you have blue buff early game. Once you have frozen heart, you should be letting others have it, as much as the mana regen is nice, you really don't need that 1% extra CDR.
As for items after that, it's really your choice. All I'm going to recommend is something with magic resist and something with more damage output. For my next 3 items, I prefer to pick banshee's veil for the mana (more damage, manamune), health, and magic resist. I'll take a sheen for some nice bursting. And my last item would either be bloodthrister if they decided not to build armor for some reason, and last whisper if they do. The issue with bloodthirster though, is that lifesteal doesn't work with your W spamming. Infinity edge might work well but it's quite costy, like the rest of the build already.
Maybe with the item rework, you could throw in the new black cleaver.
As far as summoner spells go, I've been enjoying flash and cleanse.