The glass, and the four holes you cannot add later
Shower glass is safety glazing, which is not a preference. Glass in and around a bathing area sits in what the code calls a hazardous location, so it is tempered, and on a heavy frameless panel that tempering is what makes the piece safe to stand next to.
Tempering is also why the measuring matters more here than almost anywhere else we work. Every hole for a hinge, every notch for a handle and every cutout for a bottom guide has to be machined into the glass before it is tempered. Once it comes out of the oven the piece is finished. It cannot be drilled, trimmed or eased, and a panel that is a quarter inch too wide is not adjusted on site, it is reordered.
Thickness follows the build. Frameless doors are normally three eighths or half an inch, because the glass is the structure. Semi-frameless and framed panels run thinner, because the channel is carrying the load instead. Half inch glass looks and feels heavier, and it needs hinges and backing rated to hold it.
On the water spots: the coating worth paying for is applied at the factory and fused into the surface of the glass, so it is part of the piece rather than something sitting on top of it. The bottled sealants sold for the same job wear off and need redoing. Either way, a squeegee does more than either of them.
Tempered, always
A bathing enclosure is a hazardous location under the code. Annealed glass does not belong in one.
Holes before the oven
Hinge and handle cutouts are machined before tempering. Nothing can be drilled afterward.
Backing before the tile
Frameless hinges need solid blocking behind the wall. Easy before the tile, expensive after it.