Germany Work Visa Arrival date

[davaor]

Hello

I was recently granted a german National Visa type D. It is a visa to start working as an employee for a company that sponsored me. I requested a start date based on the estimated processing time, however it was issued in a record time. So the visa starting date is December 15 2017.
The company would like me to start working as soon as posible and if I knew the visa would be processed so fast I would have chosen an earlier starting date. So I have a few doubts
I am from a non UE country that doesn't require visa to enter Germany as a tourist, but it requires visa to work
Is the visa starting time only applicable as a starting date for work, could I arrive earlier to settle and find a place to live, given that I don't require visa to enter German soil.
Is there any way that I can start working before the visa starting time, would it be illegal ?
In case is not possible to start working before the visa starting date, I would like to arrive to Germany earlier, as a tourist, if I do this, do I need to get out of Germany and then reenter with my working visa ? Is it any difference in the stamp I get at the immigration counter ? Can I change my status form tourist to worker after I have entered Germany

It would be of great help if someone could give me some advice about this

Thanks all !