Years ago, I replaced my exterior front doors (opposing pair). They were a frame and 6-panel construction. The panels were very loose and water was coming in during rains. We replaced them with fiberglass doors with stained, simulated wood grain. The inside looks great, but the outside looks terrible. The provided stain is not durable and I have had to reapply three times in 15 years. I'm about ready to paint them with an epoxy-type paint for better durability to the sun.
When they were installed, they were hung on the existing casing. The installer used a power plane to fit the doors to the opening.
You will need to install the vertical up/down deadbolts for the secondary door. This will require drilling and/or mortising in the top and bottom edges. You will also need the overlapping door trim for where the two doors come together.
If you are used to using the sidelights for checking who's there before opening, you might want to consider doors with windows at the top.