I've dealt with this topic. Bought a place that had two shower windows. Previous owners had tried epoxy paints on the old windows. The corners still trapped water and failed from water leaking around and into the framing. Frames were turning into mush. Water had leaked behind.

Someone also had the brilliant idea to slap tile right on the old walls. It was a bad combo.
This was in a 100+ year old building with stucco exterior. Believe me I was nervous. I didn't want to start goofing around with the opening and redoing the stucco. I also had a building that stood fine for that many years without problems except the showers. No reason to screw up that history. I knew that slapping more fancy paint on the windows wasn't the answer.
Knowing what I know, I wouldn't worry so much if I was doing it again.
Pull the trim, measure the opening, and go to H.D. and order a tempered glass sealed unit and be done. I selected vinyl framing, but the fancier stuff will work too. Check to see lead time and pricing.
I did tile surrounds in both those showers. I used a Kerdi system for tile after sealing the opening and installing the window. I tapered the interior bottom sill inward and used high quality sealant to the window.

Whatever you decide on the shower surround, plan on the window being part of the shower. Synthetic window framing is already exterior material. The smaller opening makes the selection easier. Expansion and shrink won't be a reason for issues if it's done right.