I used to play some chars with different classes and races but still didn't manage to get beyond level 75 and therefore wasn't on Pandaria yet. After a few months I wanted to create a new char and would like to play as a Pandaren. What are the differences starting as one in contrast to level up and get to Pandaria? To what extent am I spoiling myself about this extension and the corresponding regions?
They are 2 totally different things.
The Pandaren do not start in Pandaria, they start on the Wandering Isle and do not choose if they want to be Alliance or Horde before they finish the quest line on the Wandering Isle (about level 10).
Once they have chosen their faction they find them self in either Stormwind or Orgrimmar and can progress just like any other race/class at that point.
No matter what race/class you choose normal progression does not get you to Pandaria before level 85.