I am assuming that you have 4" steel casing with threads up! If this is the case I would recommend installing 4" steel and a coupling. The other option is to use 4" PVC and a female adapter with a threaded bell down. Screw it onto the steel casing.
Then I would install a 4" Pitless adapter (such as Merrill's MCK Brass Pitless Kit) below the freeze line. Keep it simple! NOTE: To adapt to 6" pipe can present a problen in that the pitless won't allow the drop pipe to line up in the well.
I'd go with a 4" submersible if it will go in the well. 3" submersibles are OK but don't usually last as long because of the speed they run.
I'd also recommend installing a Cycle Stop Valve in the line to extend the life of your pump and give you constant pressure like city water. It's not expensive when changing to a submersible.
Just be careful not to drop anything in the well!
I understand needing to save $$$$ but the above should be done by a licensed Drilling Contractor. I don't know where you are in MI but I suggest that you find a Preferred Pump Supplier in Lake City, MI or Milan Supply in Mt. Pleasant, MI. I'm sure that they will help you. . . Just tell them that "Porky" sent you. Note: Don't let anyone sell you a computerized pump because they are a nightmare and expensive!