Sounds like you're considering a big undertaking! It's difficult to say exactly how much it would cost to add a second story to a home. There are many factors involved. Your best bet would be to consult with an Architect or Contractor to try to get an idea of the overall cost. Other things to take into consideration are permit costs, zoning restrictions, etc. Also - is the house serviced by public water and sewer? If not, there could also be other restrictions as far as what you are able to do.
I have a great network of local professionals that I work with that would be happy to answer any questions you may have. Best to get your questions answered before you commit to buying that particular home.
Feel free to email me firstname.lastname@example.org, or give me a call at 215-740-0607. I'd be happy to refer you to the right people to get your questions answered.