I havn't downloaded them yet (earlier today the Store didnt accept my code yet), but I suspect that the DLC cars are already on the disc, and is handled in a similar fashion to costume-dlc in LittleBigPlanet. Everyone has them, but are not unlocked for the user to play yet.
They might also be included in the 1.01 patch that will prompt when you insert the disc in your internet connected ps3. The 1.01 patch is 113 MB.
But you want to have ~ 10Gb of free disc space on your PS3. The gamebox mentions about 8gb, and a clean HDD install of GT5 takes up about 6.5Gb.
