@Eric Carlstrom Cast iron is fine until it isn't lol. I have a 1947 that is fine and then I have a 1912 that needs completely replaced. I bought it in 2013 and it was fine fast-forward to last week and I've got a pipe from the laundry room that is rusted in two pieces and dumping under the house. I've owned the 1947 since 2008 and those pipes look like they were put in yesterday. The difference in the two is the amount of moisture in the crawl space. The soil is always slightly damp under the 1912 house due to the water table being inches below the ground surface. I know that was a lot but my point to the story is the 1947 pipes will probably outlast me because of the environment they are in. Your home inspector should be able to give you a good idea of the environment under the house along with a visual of the pipes. If they have the original paint on them I wouldn't worry near as much. If they look like they are rusty and you would need a tetanus shot if you touched them then I would worry a little more.
Other than that everything Lara said is spot on. I personally worry about the area around windows and Fascia/soffit areas for wood rot also.