There would be quite a few people interested I would say.
You are more likely to make a worthwhile business out of bodykits and the like, or a range of unique custom bodykits that aren't available anywhere else. Most of the bodykits available in Australia for Skylines and most of the Nissans are of about 5 different types. Pretty poor selection, and to get something different requires a lot of money and/or shipped from overseas.
Fibreglass or carbon fibre of course are neato but there are legal conisderations for the end consumer. Replacing a factory metal panel with a fibreglass one is technically illegal, and if involved in accident, insurance may not pay out, etc as the structural integridy of the car is reduced. To be honest I'm not sure how comfortable I would be driving around in a street car with key items replaced with fibreglass. Would you trust a fibreglass door to save you if somebody t-boned you at speed? Of course that probably isn't your problem, but may prevent potential buyers.
That custom carbon mob can make anything it looks like anything, so you would be in competion with them and established places such as UAS.