As a prospective second wife, should I ask about my future husband's property and estate planning?

Yes; asking politely before marriage is mature preparation, not a sign of being money-minded. You need to know the real picture: whether he can support two households, where you will live, and whether that home is in his name. Understand too that you will share the wife's portion under faraid with the existing wife, and the home you live in may become part of the estate later. Ask whether he plans to write a will (wasiat) and update his nominees. Put your questions in a calm tone, for example: ‘I want us to be fair to everyone, including the children in the other home.’ His answer will tell you a lot about how ready he is. See prospective second wife.