Building a city in Unity?

Hi I plan to make a small game in Unity. The map should be made from real-world heightmap data (of course a really small map otherwise it would take forever to make). My question is: Would it be best to make everything in Blender (Terrain, Houses, Roads) and build the map IN Blender - or should I make the models in Blender and then import them individually and build the map in Unity?

First, to make terraindata out of heightmap, you should study the heightmap then make it, or just get WorldComposer.
Second, I think you should make terrain and roads in unity(for roads use easyroads3d free), and then make several models, basic buildings of various sizes and shapes: City homes, Rural homes, Townhouses etc. Then models the cities landmarks, and then place the buildings on the map. Note: It should be in small parts so you can somewhat re-use several buildings without them looking like clones.