What type of Mobilis pass would I need to buy to get from the Gare d' Est on the Bus 350 to Charles de Gaulle Airport? I have lots of time and the long bus ride is OK with me. I just wanted to try this since I visit Paris often. Can this be done with a zone 1-2 mobilis that I would use earlier in the day?

You'd need one that is good in zone 5. No, you can't go to CDG with only a zone 2 pass. You can't go to any airport, that's only for central Paris.

In fact, I guess you know this but the regular bus is only method you can use a Mobilis to get to an airport. You can't use it on the RER or Roissybus. Buying a zone 5 pass for one day is pretty pricey (17 euro), it would be cheaper to just buy an RER ticket for that trip.

Buy a carnet, use the tickets needed (5?) and save the others for your next trip.

350/351 require 3 tickets t+ to travel the distance from CDG to Paris or Paris to CDG. Three tickets t+, even at the individual price of 1.80€, would cost a lot less than purchasing a Mobilis pass.