For what possible purpose do you want to use rubber bullets for? If you're buying a gun for self-defense but are worried you're going to kill someone (who is probably trying to kill you) then you're best to save yourself the time and money and just buy a can of OC spray. Guns are called "deadly weapons" for a reason. However if you're serious about self-defense, you'll flush the rubber bullet idea, and all the great information from TV shows, out of your brain.
As for caliber, my suggestion is 9mm. Recoil is manageable for a new shooter, ammo is fairly inexpensive, and it's more than adequate for self-defense if you use a premium defensive round.