A lot will depend on how precise you wish the car to look. If extreme precision is demanded, you are probably going to have to employ "Grant Line" plastic bits and pieces to give the flat car that finished look. (Stake pockets, air brake underside goodies, end sill stuff, etc.)
Based on costs, most folks scratch build to high detail only those cars that can't be had R-T-R or as kits. Odd ball, custom length cars or custom purpose cars are often just kit bashed.
It is often advantageous, cost wise, to get a base level kit or R-T-R model of some flat car and repaint, redecal and super detail it.
Until recently, almost all narrow gauge cars were only available as kits and in limited numbers, at that. Thus, narrow gauge folks were always scratch builders and that trend remains today among the most committed narrow gaugers.
A total scratch built car usually heralds ultimate detailing and other special features just not to be had otherwise via any retail or kitbashing effort. All the best of luck in your effort.