Searched and didnt find what I was looking for. two questions:

1. what offset 18's will be needed to clear ls400 brakes?

2. what are my aftermarket pad options?

 

1) offset is not the key. the spoke design is. there is no definite answer to this
2) i don't know much about pads for that caliper

 

I dont quite understand how its spoke design and not offset, can you explain?

 

sure. first of all let's not consider offset (assume you have some wheels of the same widths and offsets). on the first on, the spokes are very straight and flat. that doesn't clear your caliper. but on the second one, the spokes curves out from centers, all the way out, and then curve back in. that creates the necessary clearance to clear your calipers.

that's what i am talking about, and in most cases, that's called a high disc wheel setup, especially for jdm wheels. keep in mind, the spoke design does NOT change the offset of a wheel at all.

for example, i have ap racing and soptech brakes on my gs400 and sc430. both the wheels on my car, the front spokes they curve out a lot and then goes back in to create the clearance, unlike the rear where it's a lot flatter

 

Gotcha