Make sure you wash all the parts in luke warm water with washing detergent. This will remove the release agent from the plastic.
Yes prime before painting. I like Tamyia rattle can fine grey primer
The insides will depend if the outside has loze on it. If ir does you could use loze but applied with loze painted side to the fuselage inside. That way it will look like the loze for the outside showing through...
Or if it has no loze you can do the insides in a cream colour (CDL)