You said that you’ll be taking over a property with a $450k debt & it’s worth approximately $900k.
Even if the approximate value is overly optimistic, there is still a lot of equity in that property. You’ll be making a lot of money even if you sell it at $700k.
The current owner is not stupid (she maybe naive about the market), but stupidity is a different matter. Why would she allow you to do that ?
