I'm having a well drilled 750' deep; my pump is a 3 HP, 10 gpm submersible unit. I think I need to use galvanized steel pipe rather than PVC because of the depth and weight of the water. I've figured 35 joints of 1-1/4" steel pipe; but I don't know what schedule the pipe should be. Once I know the pipe schedule, I can figure the weight of the pipe, water, electrical cable, pump and motor and then size a pitless adapter.

Foolish person. You have a 750 foot well and you come to a DYI site for advise. Consult a professional well man. Even Schedule 80 PVC is limited to 600 feet. Schedule 120 might do it. There are other factors such as pump pressure on the pipe to consider. Consult a professional and get a good one. Find one with experience at that depth.