First off, in reference to th niche, it depends on whether it's being built on site, of if the installer's using a prefab niche, as shown in the following pictures:

If it's built on site, it needs to be framed out prior to cement boarding. If it's a prefab unit, as seen in the pics, it's installed afterward.

As for the pencil rail and chair rail, those are decorative pieces that really aren't needed at all. They're strictly aestheic pieces. One thing you want to keep in mind-- if you're going to be using a glass enclosure in your shower, you want the wall to be completely flat where the glass will butt up to the walls.

Thanks, where can I get a prefab unit? How high should it be?

The name of that particular unit is called "Recess-it". The link to find the nearest distributor is below. As for the height, I'll usually set it give or take with the bottom of it at about 48" off the finished shower floor.

Here is a link that might be useful: Innovis

I'll order that unit for the niche. What should I use to interface between the tiled and painted part of the wall? For ceramic tile there's bullnose edge tiles but can't seem to find the marble equivalent. Thanks

For marble, either your installer would have to polish or bullnose the edges himself, or you'd have to bring the pieces to a stone shop to have it done.

Swanstone also makes a prefab niche that sets over top of the tile. We've used them in several showers.