A well dug with a mini-digger suggests it is not very deep and therefore it will be very difficult to keep unwanted nasties out of it.
A sulphur smell is probably hydrogen sulphide of something similar. Probably caused by bacteria in the water.
I had this in my water supply, I think because the pipes sat empty for months, the water in the taps was coming out smelling of sulphur. The solution was to pour a bottle of bleach down my borehole and then flush a lot of water around the system to make sure it went through all of the pipes and was properly flushed to again (as bleach in the drinking water supply is not so good!)