Creating a building game

I am developing a sandbox-style game where automatons construct a multilevel building in real-time. The player would view the building from overhead. The building would be composed of thousands or even tens of thousands of parts; the more the better. Real-time collision is necessary but could be approximated; no other physics is required. What could I do to make this system efficient? I am well aware there would be significant overhead in spawning tens of thousands of game objects. Can I during the game merge individual parts into large (but custom) meshes to reduce overhead? Am I going to have significant lighting challenges? Is this possible within UE4? Any advice appreciated. Thank you!

Duplicate of: Creating a building game, need suggestions an an optimal approach - Debugging, Optimization, & Profiling - Unreal Engine Forums