During the 1960s and 70s aluminum wiring was often used instead of copper wiring. If you find this in your home, you will likely need to replace it as it was found to be defective over time.
Asbestos is now recognized as a very hazardous material, however, during the 1930s and 50s asbestos insulation was commonly used. Don’t try to remove this yourself, it should be taken care of by a qualified professional.
In homes over 40 years old you may find lead paint is a problem. Lead paint will become a hazard, particularly for babies and small children, if it is deteriorating and chipping or turning to dust.
Radon gas cannot be discovered by smell or sight, but your old home may contain higher levels of it which can be harmful to health. A test should be done to determine radon levels in the home.
These are a few of the hazardous materials you could find in your fixer upper, especially if the property was owned by the same person for many years. Make sure you have enough room in your budget for the removal of such materials as they should be taken care of by professionals.