They can be reused, especially the condenser and evaporator. In fact the condenser and evaporator should be flushed/cleaned when the system is apart for service anyway. The only component you should not reuse on an air conditioning system after being exposed is the receiver drier as this contains dessicant and it will continue to draw moisture out of the air making it useless in the car. Also if you are doing a r134 conversion I would strongly recommend getting new hoses designed for use with r134a as these hoses have a barrier layer which helps prevent the smaller r134a molecule from permeating through the hose. Also make sure you get a receiver drier and orings which are compatible with the oil being used in your r134a compressor. It sounds to me like the mustang shop wanted to sell you some of his excess a/c inventory.