What to look for in a dog that would be suitable for my mother?

You want a dog of decent size of course. He needs to watch you. Follow your every movement, perk his ears at every sound. Not react to all these things, but be aware of everything around him. You mentioned that your mother is infirm, you might want to get a pro trainer and bring them with you to get the dog, that he or she will train, to take care of your mother.