I agree that you should have some blood work done first and then find out what kind of anesthetics will be used. Some, like Ketamine, can be contraindicated in cats with kidney problems. Others, like propofol and isoflurane, are much easier on the cat's body and the cat recovers more quickly. With anesthetics, there are often three or more components: preanesthetic, induction agent, and maintence anesthetic. Make sure some sort of pain relief is given both during and after the surgery, especially if there are extractions.
My 15 year old adoptee has had dental surgery twice (I need to get back into the habit of brushing his teeth, or he'll probably need it again) and my previous senior, Spot, had a couple of surgeries with the not-so-good anesthetics. One of those included a dental. Spot did fine despite heart disease and hyperthyroidism. One of my boyfriend's cats has had dental surgeries and extractions as they think she has stomatits. She's also around 6 pounds.
As long as your vet is experienced in doing dentals on older kitties, it should be fine. Not cleaning the teeth can lead to bacteria getting into the blood stream and damaging the heart, kidneys, and other organs. I would try to get into the habit of brushing Simba's teeth too, as that can extend the time needed between cleanings.