Hi Jin and welcome to the forum,
I can confirm that there is no steel laminated into the structure of the Cortez, or any Midas for that matter. The only parts that give us rust concerns are the subframes. The rear ones, in particular seem designed to trap road salt. That is why I have started getting good subframes galvanized to give them a few more decades of life, hopefully.
In answer to your question about the Excelsior, the structure is beefed up with a thicker floor and sill sections. That's the great great thing about GRP, you can just add a few more layers to increase the strength where you want it. In the case of the Excelsior that includes extra core-mat in the floor to increase its thickness - more glass means more strength, whilst more thickness makes for more rigidity.
The Gold convertible has a foam sandwich floor for the same reason. The foam is lighter than core-mat, but harder to get the construction right.
Hopefully you will get a car soon and find out just how great they are to drive.
Regards,
Alistair