Having had personal experience with this, and done a lot of research, I'm with those who say that the danger is overblown for most people. The exception is the small number of people with serious allergy problems.
If you and your family are not in that category, then what you have is not a mold problem but a moisture problem. The mold is just an indicator. You need a good inspector or contractor to tell you what is needed to correct that, and then make your decision about purchase. Personally, I'd factor that into the price rather than try to force the seller to fix it. That way I'd have more control over the fix.
Once the moisture problem is fixed the mold goes dormant and eventually dies. You could spray it with bleach or other chemicals, but since it isn't in your regular living space I wouldn't bother.