Miscellaneous Equipment
Auto Refueller
Not an essential item, but refuelling manually can become a bit of a chore on large ships like the panther clipper
Atmospheric Shielding
Travelling too fast through a planet's atmosphere without this will damage a ship. It is possible to land and take off from planets by travelling very slowly and using time advance. But it is a lot easier to buy atmosperic shielding.
Cargo Bay Life Support
Essential for transporting slaves or live animals. Flying through space without it will turn slaves into fertiliser and live animals into animal meat. In both cases this is a major financial loss. Economists might argue that animal meat should be more expensive than the live animals they come from but not in this game.
Extra Passenger Cabin
Required for transporting people around. They take up quite a bit of space, so it is not recommended to have them unless you are running a taxi business.