The Aston Martin should be in the base game if I remember correctly, just remember to check the RGT class for it. The Porsche in the RGT category was a pre-order incentive, the other three RGT cars should be included in the base roster.
So far the new Group B RWD Porsche 911 and Lancia 037 have been good fun to drive. I am fairly abysmal at RWD on the gravel or snow, but even the Group B RWD cars I don’t mind driving on the tarmac rallies. Though, apparently, however I have not tried it myself yet, the Lancia 037 is pretty manageable even on the looser dirt surfaces this time around.