1991 Nissan 300zx Drove Work One Morning Everything Fine Get Out Open Car Door

I drove to work one morning and everything was fine. I get out of work open my car door and as soon as i pressed the clutch peddle it went all the way to the floor.I left the car there came over the weekend and took a look. The Master Cylinder had no break fluid.So i put break fluid and tried bleeding the line for air maybe. But the slave cylinder was leaking. I bought a slave cylinder and towed it to a mechanic and he said thats an easy job. Looks under my car and says that its not only my slave its somehting else in the clutch and they have to open it up. its 600 he said. I never been to this mechanic so i dont know if i can trust him i am trying to get all the info i can before i go ahead with the process. he didnt install the new slave pump i had he said he will just do it all together with the trans. Any clue what it can be? thanks.