Transportation to Paris

Oh my I'm so confused! We are staying on a barge in Meudon-sur-Seine (zone 3), in July from Sunday through Saturday.

1) What is the best deal for a transportation primariily going into Paris each day? Single tickets? Visite Pass? Navigo? Mobilis? How much should we expect to have to pay?

2) How much is a typical taxi ride if we want to go to Meudon-sur-Seine from the Eiffel Tower or Notre Dame late at night?

3) Is Meudon-Sur-Seine safe at night?