Yes, you can turn off Cellular Data in Settings > General > Network > and switch Cellular Data off. You can use WiFi while not on Cellular Data, too
Be aware that whether Cellular Data is on or off, YOU ARE STILL BEING CHARGED FOR CELLULAR DATA, because you have a Smartphone, which needs a Mandatory Data Plan. You can turn it off to save data, but it doesn't matter much.
Yes you can, but keep in mind that you will have to buy a data plan with a voice plan regardless whether you want it or not. Unfortunately, with iPhones, there is no such thing as a Voice Only plan for iPhones.