In my boat building, I would use no finer than 220 for that, and probably 180, saving the finer stuff for finish coats. I found that something like 320 just burnished the surface. I'd think it too fine for getting rid of brush marks.

To answer a second question- I have contact cemented a formica disc to a foam pad on an ROS sander (Porter Cable 7335) when I wanted REALLY flat surfaces, such as refinishing a table top (when I had the furniture refinishing shop). You need to be careful to not tip the sander though- it'll dig a trench fast!!