Sorry to hear about your shower pan failure -- I feel your pain, I discovered the same exact thing about 3 months ago. You got some good advice from johnfrwhipple -- open everything up and expose all the rot, even if it means tearing out some drywall that looks fine. This is like dealing with cancer, you have to cut into some of the healthy stuff to make sure you get all the bad stuff out. You will be glad you did, and it will not add much to your cost for a few extra sheets of drywall and some 2x4's when you go to rebuild.
Take pictures as you progress. As long as you're not dealing with any load-bearing posts, all you have to do is replace what is badly damaged with a new, exact replacement. Keep an eye out for termites, they love this sort of situation. But from what I can see in the photos, I don't see extensive wood damage other than mold. Let everything dry really good and then do a close inspection for unsound wood that is falling apart.
As for rebuilding the shower, I am in the process of putting in a prefabricated unit (a Sterling model). Yes, there are lots more options when you build with tile, but after I ripped out my barely 9-year-old tile shower and saw how just a few cracks had resulted in so much damage (damaged 2x4's, drywall, hardwood flooring, termites, etc.), I saw just how many ways a tile shower can fail. The prefab units -- there's just a lot less to go wrong, no grout to struggle to keep clean, and much more economical/simple to install.
If you do choose to install a new tile shower, make sure it is done RIGHT. The problem with mine was that the "curb" of the shower (the part you step over to enter the shower) was simply a 4x8 laying on the slab, covered in black paper, then sheetrock, then tile. Holy smoke! Any fool can see something like that is an accident waiting to happen. All you need is a small unnoticed crack in the grout and then water weeps in and next thing you know, the wood is expanding and the tiles are cracking and the grout is falling out and that's when the termites move in. If you look through this forum and other forums online, you'll see example after example of poorly built tile showers failing like this.
God be with you.