Builders would be passive units that would be bought from all factions. They would build structures specific to their faction (if given the appropriate materials) for instance if you hired a Rohan builder it would only be able to create Rohan structures. They would cost 30 coins.
When you right click on a hired builder a screen will show up displaying the createable structures (for instance a gondor builder could create a gondor town, gondor watchfort, etc.) When you choose a structure another screen will come up showing the required materials, for instance 2 stacks of Oak Wood, 3 stacks of Gondor Brick, 4 stacks of Oak Wood Planks. You would need to get these materials your self and deliver it to the builder.
As I stated earlier depending on what kind of builder you buy you will be able to make different structures. For instance Rohan Builders make Rohan structures (Mostly structures that can be built using the build structure item in creative)
The Builders would have an internal clock so that if you were off exploring or you were not on at all, they would still build. (If an internal clock is possible for NPCs)