We are arriving on 1st June,17 ie Thursday morning. Is it advisable to buy Nevigo week pass along with Navigo Découverte pass? How and where to buy it? to buy it?

If staying in Paris 3 or more days and you plan on using Paris Metro and RER trains several times per day, then Navigo is a good options.
 
You can purchase Navigo from any Metro or RER or Transilien ticket window.