Title through me off as you are replacing the impeller not repairing it.
For lube use quicksilver high performance gear lube.
Not sure if you mean the gear lube gasket that seals the lower and upper lube passage, should be in the kit. Small round flat washer gasket.
Tips gear shift must be in forward to disassemble and assemble. The lower unit must be in forward when installing, applying slight pressure by turning the prop shaft just enough to ensure load is kept on the clutch to keep it in gear. Check that you can shift from forward to reverse with engine off before filling gear lube in case you need to separate the halfs to fix an improper install.