Mountech was released in 1983. Check inside the derailler unit for any writing or letters which will allow for identification of the exact date the derailler was made - unless someone else knows by looking at at. I would suppose that the bike is around 83-85 model, if the derailler is in fact the original equipment.
Maybe do a yahoo or google search on bianchi - suntour mountech - to see if any other bikes came with this equipment. Maybe this will give you more of an idea.