Make either cars dynamic or all lights. Or turn off lightmass for them. This way Unreal will not create lightmaps for cars.
For reflections, use world coordinate materials, it will have stretched textures, but i think reflections will work.
And last but:
proper way is to create low poly meshes of those cars with uvs and render normal maps. Then create proper skins/materials in photoshop or substance designer.
Depends on number of car models, if its just few then improving them is worth the trouble but with many it depends on how good result you want.
I would make one perfect model for comparison, then play with low quality model and clever materials and compare results.
PS. If you have (or plan to have) any real texture on those cars, it will get stretched without proper uvs there are no miracles. This worldd coord uvs can only work with flat surfaces or some reflection only materials. But you really want normal maps for models, and that requires hi and low poly model and uvs.