Interesting idea, this reminds me the Settlers game series. This is on no roadmap I’m currently aware of, but you can add it yourself if you need that. You should be able to adapt the ConstructionSiteComponent by tracking how many resources have been brought there, and don’t start construction until all resources are present (bStartImmediately = false). Let me know if you need any further assistance.
Not that I’m aware of 
Strictly speaking, walls are just buildings with any additional components (e.g. ConstructionSite, but no ResourceDrain or Production). Building nice walls would be more on the UI side of the game, like dragging walls and queuing the build orders for all of these walls.
Do you have a C++ Unreal project there? We don’t ship the plugin libraries (yet), so you need to compile them yourself.