Question from a Rolex newbie

[0037ksm]

Hi

I recently purchased my first Rolex (yea!!). It's a Submariner with blue dial in stainless and gold. It's beautiful watch and one I have wanted for many years and I have finally reached a point in my life where I can afford such luxuries.

My question is regarding the serial number. Mine starts with a "D". My understanding is the new ones start with a "Z". Are there any important differences between the "D" and "Z" models or is there any other reason I should insist on a newer "Z" model. After spending this much money, I need reassurance I purchased wisely.

Thanks in advance for your advice and counsel.
Gary