Coaches are still a significant area for scratch building as a coach is generally a smooth faced box with windows. Bogies are readily available as are underframe components.
There are many coach kits available, some in injection moulded plastic and some as etched brass kits. They tend to be relatively expensive and when you need 8 of them, scratch building suddenly becomes quite attractive.
Of all the model areas provided by the trade, only a few types of coach can be bought as kits or ready to run, compared to locos and wagons where there is a huge choice. Some reviews of ready to run coaches are available.
Some guidance in rolling stock construction is available as part of the guild manual.