There is gameready low-poly sci-fi assault drone with PBR textures. Equipment consists of 4 engines, one machine gun and 4 missiles.
Unity and UE4 engines allow to use packed textures to optimize memory usage (for example, in UE4 textures, metallic map is packed in B channel of OcclusionRoughnessMetallic texture).
For any questions related to the model or problems using the UE4 and Unity textures, you can ask me.