To fit 255/35 18" tires, what offset would I need if its a 18x8 wheel?

You need an 8.5"+ wheel to fit 255's and an ET35~39mm offset with rolled front fenders

Actually, for an 8.5" wheel, use an offset between 35 and 38mm. For another width...
 

...the offset will be a different range, for ideal fitment.

255's are probably too big for 8" wheels.

HRE claims ET39mm will work fine for their 8.5's...*shrug*... you'd prolly know better tho ;-)

will these offset clear aftermarket brakes?

Yeah, 39 would probably work, but that's cutting it real close. :-)
 

I figure 38 is as high as you could go, allowing a little room for 255's from different manufacturers.

And just to clarify (for others), the lower you go (like 35mm or lower), the more rubbing you'll have on the fender.

Depends on the wheel design.