Indeed, pcars2 is what the 1st should have. There may be a few different cars, and many are small variants if the same cars than pcars2, the karts are different (a 125cc with shifter and a 250cc superkart), but overall less tracks and less cars. FFB is not easy to set, once you found good parameters you have to duplicate them for each car as part of its setup (you can only save one by car).Tracks have better tarmac textures though, much more detailed. I've read the career is much better in pcars1, I can't confirm as I don't use the career.
You should not need pcars1 except if you find a car you like and not available in pcars2. Excepted the karts and the japanese pack, I don't have any other in mind.
You should compare the cars and tracks lists (I think all the pcars1 tracks are in pcars2) and decide whether they worth the cost (it is regularly on sale for around 8$, not that much for a few cars you are interested in).