Digging anything, even a trench, which is deeper than 3 or 4 feet is dangerous and you have to support the walls adequately so they do not collapse. It would be extremely dangerous and foolish to start digging a well without knowing exactly what you are doing.
Couldn’t tell you as I’m not familiar with water tables in your area. In New england it is very rare find a static level below 30 feet 15 feet is very common. The average well depth to attain enough flow for most homes(4 gallons a minute) is 130 feet. As I’ve mentioned before it still comes down to educated guesses.
An example of unpredictability is the well I was working on friday. I’d worked for the customer at his former house three lots down. He bought his new house and needed a well for it. Both houses are lake front property. His old house when we did the well we stopped drilling at 200 feet finding at that depth the well could produce 15 gallons a minute. We drilled at his new house to a depth of 600 feet finding it only would only produce 3 gallons a minute. As expenses for equipment get much higher after 600 feet we decided to stop there. We had the well hydro fractured to try and increase flow. It went up to and maintains 4 gallons a minute.