I'm not necessarily talking about the $12 to $20 sensors. Most people get their brakes done when something is already wrong...like grinding sounds or brake shudder. Now your looking at rotors, pads, labor, it adds up quickly since I believe you cannot turn the rotors on a Porsche. (Not sure but this is what I have heard).
What I am saying is if you wait for the brake light to go on and the previous owner had this sensor removed you will be waiting for a large repair bill since this light will never go on to warn you. Get the pad thickness measured to be sure of an approximate time before they are due for a change. You can measure them yourself very easily if so inclined. This gives piece of mind instead of just "waiting for the light" and you can change your own pads VERY easily!
In short...the light should ONLY be there to be a final warning to you to change your pads. YOU should be aware of your cars needs before the car finally tells you something is wrong.
BTW-After you have changed the first set of pads you will be amazed how easy it is to do on this car! It will take longer to take the wheel off than the change the pads!