Build it all out of Corian please.

Pvc will be fine. Once painted you won't be able to tell if it's wood or pvc. Make sure you get tempered glass for the window above the tub (the plexiglass interior storm window might suffice, check with your building dept.).

You won’t need any plexiglass if this is constructed properly. Right now, it isn’t sounding like it’s being planned properly. What waterproofing system is in the plane to be used? Kerdi? Laticrete? Wedi? Be sure to integrate the shower waterproofing membrane into the window properly. It will need to adhere to the entire wall, and fold into corners of the window to adhere properly to be sure there is absolutely no exposed part of the wall that allows water behind it to rot your wall. Then use tile trim, like they did in vintage tile installations, if you want a clean vintage look. Or use Fypon if you want to have more nooks and crannies to clean.

white pvc trim works nice and can be left unpainted. It comes in many profiles and is good solution when not replacing any of the tiles and retains the traditional look.

A Picture would help...

Also you may want to consider something like this:

We are re-tiling the shower (and we want this tiling job done properly, so we are having the current basic, cheap vinyl window replaced now so that we know it's going to last with our tiling job). It will all be properly sealed using waterproof materials by the tile contractors. The plexiglass is just an added layer of protection for the window itself, and for the corner where the window meets the sill to not be inundated with water daily.

Regarding a picture -- my window placement is exactly like the one in the link I posted in original post.

I am primarily needing to know what materials one might suggest I shop for to use in the window recess/sill. Thanks! I really like the way that window turned out in their remodel, and the blogger said they did not use any trim pieces for the corners. Wouldn't the raw edge of the tile be obvious?

IT looks like in the picture it is a solid piece of marble for the sill and then the inner part of the window is a bullnose tile. That tile will meet with the tile on the wall and overlap - the bullnose finishes the edge of the wall tile...this is not the greatest picture but you can see the bullnose tile

You need to be comprehensive here. Tile and window guy must be on the same page.

If you are going to replace the window anyway, install a vinyl window and eliminate the plexiglass shield.

You can trim the window with the same tile you use for the wall. Make sure the sill slopes toward the shower so water will drain. Use a waterproof cloth under the tile, then it does not make any difference whether the substrate is waterproof, water will never get to it. We use Kerdi from Schluter, but there are others.

This results in a completely waterproof system.

We rebuild a lot of old pre-1960 tub-showers with windows, always do it the same way and have never had a leak.