If the aircraft has an CofA, then you are not legally allowed to change seat belts unless you are a flight engineer or something, and the seat belts have to be original or there has to be an STC for it.
As for the sun visors, make sure you get the right ones for the engine type you have. The PA28 left sun visor has a table on it for best power and best economy RPM settings of your engine at various altitudes.