2004 Vauxhall Zafira Replaced Man. Gearbox & Clutch inc

Gearbox (Manual 1.8 Petrol) was making a buzzing/hissing type of noise on acceleration for a few days. I was told by a mechanic that the Gearbox was on its way out. I was told by a friend that I may as well change the Clutch at the same time. So I bought a used Manual Gearbox and also a New Clutch Kit and a Hydraulic Slave Cylinder as recommended by the Parts shop. Five days later the noise from the Gearbox got worse and I started having trouble getting in the gears and then all of a sudden the clutch went. The car still drove but without gears or clutch.
1. Was the problem the Gearbox i.E. Dying or just low on Fluids and did the Gearbox cause the Clutch to go?
2. What is the recommended transmission fluid that should go into the gearbox as the Scrap yard said put only Automatic Transmission Fluid as the others are too thick and they will not guarantee the gearbox if anything else was used. Is it o.K. To put Automatic Transmission Fluid in a Manual Gearbox. Mechanics actually put Semi-Synthetic as recommended using the car registration? But can change if it is wrong!
Thank you for your time and your help.