If you do the B'tran explore and the vice admiral B'tran you will get 5 marks and 4 emblems per run. If you do the Breen/Devidian dailies on top of that, you will get another 6 emblems for a total of 15 marks every day after you convert them all to marks of exploration.
So at 200 marks, it shouldn't take long to get the nebula class. You can buy it from ESD.
^^ This... Forgot that it was different from the Retrofits.