I assume that you might be a dental student? or Dentist taking a picture for your case record. You can use the dental light as lighting, so the ring light is not really a necessary, but you can get one if you think its not bright enough. A simple kit lens can get things done actually if its just for case record. Given that you are not doing those dental branding advertisement level of shooting, a simple A6000 + kit lens is more than enough. The most important part is the lighting. Project the light nicely and you can get a nice intraoral shots.