Reading the title I thought you were undecided between a 4" and a 6" casing. 2" limits your options way too much WRT deep well pumping. I would advise a 6" casing so that you are not limited with what submersible you can use.
Filters/purifiers will of course depend on the water quality. I would start by asking the neighbors questions about their wells, how deep they are, where the water table is, and what the water quality is. Ask them if they treat their water and ask for a sample of untreated water to take for analysis.