We have a 1971 bungalow with terrazzo floors. We noticed a dark stain on the floor along the baseboard in the linen closet. The closet shares a wall with the master bath walk in shower. There is a small discoloration in the grout along the bottom of the shower wall. I have carved it out and re-grouted the spot twice and it discolors again. A test of the water pressure shows that there are no leaks in the pipes, so we have concluded that the shower pan has rusted out.We were planning a minor bathroom renovation anyway, but now this makes it a major one.
Where is the best place to start? My husband and I are experienced in tiling but neither of us know anything about building a shower stall. Will a plumber do that or do we need a renovation contractor?

Neither, a plumber can do the work but a contractor usually subs out the job. You can talk to a tiling company who do shower pans (they don't rust) or you can consider doing a complete tear out and install an acrylic pan. Since this home was built there have been may new and better installation methods than a mud pan.

I appreciate your time and thank you for the information. I'll call some tiling companies... never thought of that!