#3 you can't use other's models at all, even small bits, like lightbulbs. That would be illegal and you would be commiting a crime.
The only exception are models that are distrbuted under CC0 or public domain licences. You can do with such models whatever you want - use in your work, modify, sell, even sell without modifying. But models with such licence are very rare.
Anyway, it's always best to make sure that model is 100% made by you.
BTW, same goes for textures - you can't use them if licence doesn't permit that. But contrary to models if you modify texture enough, you are safe to use it on your models.