You think you need 140 cars in garage to have 3k D points ? answer is NO.
all you need is to have 10 free car slots and for IGC you can buy 10 of each car. example : 10 of Mitsubishi Eclipse GT - get rewards and sell it all! D points not decrease. after that buy oher 10 similar cars ... and so on.
PS. if you have a little of SB - make +100% IGC for 500SB for faster Grow up
here is car list and you can view it costs