About the strap, I would recommend to play around two topics: the strap color and the texture. It is not easy to find a color which matches a black dial but khaki works well... and gives a more casual and military (yes at the same time!) style to the watch. Grey is also a good option but less fun. About the texture, I recommend to stay with a high end strap for a Lange watch and so a croco leather remains the best option. Another one: calfskin but with a high quality.
Best regards.
Fx