My Easy Jet flight is arriving at CDG Terminal 2B Can I buy tickets for the RER B train from CDG to Paris and the billet Ile-de-France ticket for the train from Gare Montparnasse to Rambouillet on Transilien, at this Terminal ?

If not where is the nearest purchase point for these tickets. and how can i get there ?

I shall be travelling with a small case, (cabin baggage) and a larger case (hold baggage) will this present a problem on the RER or Metro (my flight arrives in CDG at 1600 hrs) Any suggestions as to how to overcome possible problems.

Which is the best interchange station on the RER for connection to Montparnasse Bienvenue ie the one that will allow the easiest transit ?

Thanking you in anticipation.