oh yep I agree with 6 and 8 too. I straightened that number plate so many times, and a long trailer can scrape on surprisingly small angles.
but I don't really agree with the galvinised thing, just paint it every few years. gal also makes it harder to modify/repair because you need to remove the coating before welding.
one other thing I forgot...if you have rails or mud guards make sure they are low! so many trailers you can't open the car door once it is on which means you are dukes of hazard through the window all the time.