First of all he's capable off going pure DPS and OFFtank.
He kinda reminds me of Jarvan and Xin. Just of Third and first skill. But he got one better thing than those 2 chars "Decoy" this makes him strong. Why? You can sneak up around just before ur tank engage the teamfight and launch up ulti to shatter enemies or just rush in E>R>Q>W and ur out of fight

Why 21-0-9? Because it's most common build for DPS and works well with off tank. But If you're wan't you could go 0-21-9 But it doesn't work so good.

Why 2 dorans? Because I like it. You can skip second doran if u don't want it and go for trinity.
Warmog and banshee give's you more survaivability.
Infinity/Blood are dependant items.
WuKong doesn't need speed items just coz of his third skill
Shoe's depending on situation

Exhaust is JUST GREAT! running off low hp that u cant get or strong dps (jax). You can shut them down easly
Flash is great too with ur W skill you can escape fast ;)

I haven't checked any other but I bet
Ignite would be nice too

That's my first build so don't be bad to me :(
Hope You like it ;) I made quite much frags with this build