D-reg. Every individual car manufactured has a unique manufacturer reg[istration] code. Beyond the parts of the code that identify the marque and model, there is a "generation" letter: A for the first series, B, C, D, and so on. D-reg means the fourth generation of the Sierra.
The "erm" is likely just a verbal pause, like "...er...", "....uh....", or "...hmmm...".
Student or Learner