shower door/wall placement, greenboard

I'm finishing a basement bathroom that was half-finished when we bought the house. The previous owner put up greenboard in the shower area, which I heard was BAD to use for ceramic tiles. Therefore, we are putting up fiberglass shower wall panels on the greenboard. Should we paint or do any kind of "waterproofing" job to the greenboard before glueing up the shower walls, or glue right to the greenboard?

Since the shower drain is off to one side, I am ceramic-tiling the shower floor (concrete), and up over the "step" into the shower (which is also made of greenboard). Should this be ok? ALSO, when I attach the shower door, should I attach it directly to the greenboard step and tile around it, or lay tile across the top of the step and attach the shower door track by drilling through the tile? I will use silicone caulk around all edges of the door.

Thanks for your time!