Unlike wood doors, fiberglass won't dent, crack, split or splinter. Unlike steel doors, fiberglass won't dent, rust or warp, either. Fiberglass also resists shrinking and swelling with temperature changes, allowing the finish to last three times longer than on a wood door. The solid polyurethane foam core offers five times greater insulation value than wood doors.
In general, we would not expect any problems, assuming that there is topcoat or paint on the door. The hydrochloric acid (muriatic) or some of the other chlorinated acids do attack the surface of the door skin irreversibly if concentrated enough (example brick wash-strength muriatic). The result is a surface that doesn't stain evenly with the surrounding surface. However, once painted/topcoated (generally done at the factory prior to delivery), this shouldn't be an issue.
Brass locksets set up an electrochemical reaction with the zinc on electrogalvanized STEEL doors. It is essentially a small battery that tarnishes locksets. Lock companies try to avoid this with plastic rings behind the escutcheon plates or application of caulk anything to break the electrical contact between the two: brass and zinc.
Fiberglass doors do not experience this reaction. Hence, no preventive steps are needed.
No, a kickplate is not required on exterior doors although it does enhance the aesthetic beauty of the door and helps to protect the bottom of the door from the outside elements and every day use.
The door works in concert with weatherstripping, the door frame, and the threshold to keep the elements at bay. Usually, the door itself has insulating value provided either by its mass, as in a solid door, or by insulating material fabricated into the core of the door by its manufacturer. The weatherstripping, doorframe and threshold prevent air penetration around the door.
Prehung doors are installed in the frame before they are shipped for installation. They significantly simplify installation. Prior to the era of prehung doors, a door was shipped without the frame or any hardware. This required a skilled installer to construct a frame, see that it was square and true, install hinges and then carefully fit the door to the frame so that it operated freely. A prehung door comes in its frame with hinges in place. It has been carefully mated to the frame, usually by the manufacturer. At the site, only a rough opening is needed. The prehung door is installed in this rough opening, a process that requires far less time and work.