Hee hee. I guess that's why I learned to do the rack myself.
Any contamination in the pump or hoses could find its way back to the new rack. I learned that old hoses can deteriorate on the inside, or swell and restrict fluid flow. In terms of parts, a new pump would run me about $100, hoses another $100 at the most I think.
If the mechanic is good, he'll at least flush the lines and pump thoroughly before install the new rack. However, I've learned that flushing only removes so much material. In the chemiistry lab, we accept three washings before we consider the fluid is a flask is fully rinsed. In the real world of hydraulic systems, contaminants can cling to hose material, and other points inside the system. Flushing will certainly do a good job and give you a good system, but I personally don't guarantee it.