If you are staying in Paris for 2 weeks, this would mean that after one week of using the Navigo Pass, you would need to buy another one ? Or you can use the same pass that you have already have and 'restock' it ? It would be annoying if every week you would have to get yourself a new card with a new photo and pay the 5 euros... Planning on arriving on a Sunday just in time to get the Navigo and save myself the hassle of single tickets and more expensive passes. :)
You would restock / recharge the physical Navigo card you have for the 2nd week. No need to buy a new card for 10 years after that.
Sunday is a good time to arrive for Navigo. It won't be usable that day, but next morning you're set. If you plan on doing many trips that first Sunday, you could go with a Navigo Jour day pass, but otherwise a single RER ticket might do the trick to go from airport to city.