I wouldn't be too worried about her behavior as they are tested before being put up for adoption now days.
While this is true, temperament testing isn't 100% and even if a dog is suitable for rehoming, it doesn't mean that it is suitable for every family and home.

On a personal note I brought home a 5 yr old large breed. He is now 12 and noone believes his age. I have the microchip data to prove it but he is so healthy and active that I am pretty sure he will be around a while longer. DS understands he won't be around forever as we have spoken about it. We will all be devastated when it happens but I think as dogs never live as long as us that it is inevitable at some point.