Should be painless. Modern dental treatment with the use of a local and possibly nitrous oxide should be pain-free.
Zero. The injection to get you anesthetized can be a little uncomfortable, but not nearly as bad as you might think. The process of getting a filling should be completely pain free because you will be "numbed up" by the local anesthetic injection. If you feel any pain during the removal of decay, stop the dentist and tell him so that he can get you more "numbed up." the dentist does not want to hurt u.
Should be Zero. The cavity itself is going to hurt quite badly some day...That's what you should really be worried about. If a dentist fixes it, you shouldn't have any pain at all. Today, most dentist pre-numb the area where they're going to place the anesthetic, so you don't really feel that part at all. Having local anesthetic is much more comfortable than having blood drawn. Fear the cavity, not the cure.