I think the amount of work that you need to put into these bottles can be assessed using two readily available tools - your eyes and your nose.
First, look carefully at them while holding them up against a light. Can you see any chemical residue on the inside. If so, John's suggestion of gravel make be required to scour away that residue.
Then, fill them with water, and then drain. While they are still damp inside, smell the bottles. Is there an odor of the former resident? If so, then you will need to do a sequence of soak and rinse steps.
But once they are look and smell clean, they are good to go. After all, you aren't going to be drinking anything from them. Or are you????