Don't feel sorry for a dog in rescue kennels!
In a well run kennels the dogs have lots of company from others of their kind - they can interact with each other. They are fed at least twice a day, they are walked often (though they do not really need it), have lots of cuddles of the kennel maids and voluteers, and have their own bed with blankets and a safe place to sleep!
Dogs prefer to be out all day running around and playing rather than shut in a house!
Please think carefully - a dog like you describe will not be happy to sleep all day and could well wreck your house (rip your sofa's, carpets, etc). It could also howl and bark all day long - do you have neighbours that would complain?
What happens if she is ill and on say antibiotics? You have to give them four times a day? What if she cuts a paw and has stitches and needs constant supervision?
To be honest eight hours a day is too long for a dog of this age to be left alone, even with a guaranteed walk at dinner time (will your mother in law be prepared to do this for the next 10 years?). Do you really mean you would walk her EVERY morning and evening? In the dark and rain? Would you get up an hour early before you go to work, when it is pitch black outside and pouring with rain?
She may have stolen your heart, but PLEASE, PLEASE, PLEASE think long and hard about this - for the dogs sake.
I would stick with your cat (which would probably leave home if you got a dog!!!).