The actual polycount does not matter as much as to how much is being rendered per frame and how many draw calls are taking place as relative to the camera so the count in polygons is not as important as to what those polygons inherent that has a performance hit and the over use of complex shaders can have a higher impact on a 100k model. Every thing else depends on design.
For example we have reserved LOD O as the high fidelity layer for menu load outs and completely dressed up player models with hair can be upwards to 200k as performance is not a concern. For real time game play, and now that there is auto LOD for static as well as skeletal objects, both the mesh and applied materials can be LODed as either a set value, based on screen percentage, or dynamically based on total render load per frame.
Our rule of thumb now that optimization can be made in UE4 is all assets needs to be at max fidelity as once in UE4 you can make something less if it’s to much rather than trying to make not enough do more. Think of assets imported into UE4 in the same way as making a high quality video. If you start with low res assets you will never get the quality output your looking for