You need to find the source of moisture that is causing the mold. Also need to know the extent of the mold. It may be something simple (doubtful) or an expensive problem to fix. One of the contractors I use is a human mold detector. When he goes into houses, especially old houses, if there is mold he begins coughing immediately. He said that 25 years of being exposed to mold has done it. In this particular case, to eradicate the moisture, he would have had to remove earth around the back of the house, excavate part of the crawl space and then seal it. It was more complicated than I'm describing, but it was also quite expensive.
Is this a house that you love? Do you have other options? Are you working with a Realtor? As a Realtor, if I'm not sure about an issue, I check with my broker-in-charge or my real estate attorney. Make sure you protect yourself.
Durham is great, lots of opportunities there. If this house doesn't work out, there will be others. If you are not working with an agent, please feel free to call me.