1. The difference between from 18'' and 19'' is nothing too significant. It all depends on what you are comfortable with because you will find 18'' and 19'' wheel owners who is fine with the ride quality.
2. We can do a non-staggered setup for you if that's what you want. However, it's perfectly fine to run a staggered setup as long as we keep those aspect ratios the same.
3. You can get non-staggered from us.
4. If you go 19x8.5 ET35mm, I would go 235/35/19 all around. If you want to do a staggered setup, you can do 255/30/19 if you have a sedan or convertible and a 265/30/19 if you have coupe.