The way I understand from the way its presented by:
When you buy lets say Supercharged now, you get all the stuff that is NOT DLC, but marked as Automation v1.0 (the first full-game release)
Some DLC will come free, other DLC will cost money (probably diesel engines for example will cost money) as there is quite some development time going into creating and calculating diesel engines.
So in short; Yes most DLC will cost extra money and you will need the full game for the DLC to work, of course.