You should at least replace the drier, and the o-rings. Both of which can be found at AutoZone. Stay away from the Wal-mart "Interdynamics A/C Death Kit"! The system will need to be evacuated since the oils are not compatible and whoever evacs the system should be able to tell you if it holds a good vacuum (if there is a leak).
If you want the part numbers for the o-rings email me. There are many different sizes and the o-rings are made by Factory air.