A racing daggerboard runs around $180. You can order it at places like Windline Sails or APS on line.
You will also need the racing version sunfish sail, if the boat you buy does not have one. They are deeper draft and significantly faster.
The rudder is standard, but racing ones are faired out on the trailing edge. Some wrap the rudder in a layer of glass.
There are some other odds and ends like outhaul and downhaul cleats, mast cleat for jen's rig and ratchet block for the main sheet to consider. Otherwise, there's not much else you can do to the boat.