If I buy Ticket Mobilis Zone1-5, I also can use all RER, Transilien trains running within Zone 1-5 i.e. to Versailles, Fontainebleau? Where can I buy the ticket and what time is the sale booth opened selling the ticket?
Yes the mobilis works for all Metro, RER, Transilien, buses (although not for CDG ORY airports) in zones 1-5.
Ticket Mobilis can bought at ticket vending machines or Metro/train station ticket windows from opening times of stations: generally 5:30 AM.