I would suggest asking that they also run a full senior panel rather than just the pre-screen bloodwork. You may want to call first thing tomorrow to see if that would need to be done ahead of time. A full senior panel should include thyroid levels as well as kidney, liver, and blood sugar levels. Also included might be potassium and phosphorus levels and a CBC. While the full panel is more expensive than the prescreen, it is a good idea for any kitty over the age of 10 and it may save you money in the long run if the blood work gives them pause. Kidney problems, hyperthyroidism, and diabetes can all cause excessive thirst and urination so it is important to test for all three.
That said, I've had two senior cats go through anesthesia. Spot had it twice for an abscess, and during the second surgery they also cleaned his teeth. He had anesthesia a third time for a rhinoscopy. Odo has only had one round so far for a dental in November, but he already needs another one (along with at least one more broken tooth removed). I would suggest asking what anesthesia the vet will be using for induction. Propofol is good but ketamine is not, especially for a senior. Ketamine is contraindicated for any cat with kidney issues, and it takes longer for them to wake up.
That said, I've had two senior cats go through anesthesia. Spot had it twice for an abscess, and during the second surgery they also cleaned his teeth. He had anesthesia a third time for a rhinoscopy. Odo has only had one round so far for a dental in November, but he already needs another one (along with at least one more broken tooth removed). I would suggest asking what anesthesia the vet will be using for induction. Propofol is good but ketamine is not, especially for a senior. Ketamine is contraindicated for any cat with kidney issues, and it takes longer for them to wake up.