Is it possible to disable cellular data use? If I buy the 250MB monthly data plan and share with my wife, but use WiFi for 98% of the monthly usage, will this work for us? We are retired and home all day.

I keep hearing that the iPhone will use cellular data regardless of WiFi, can I keep this from happening?
The iPhone will prefer Wi-Fi over Cellular data, so if you are connected to Wi-Fi your iPhone will use that connection instead of Cellular.

To completely disable Cellular data, go to Settings → Cellular and disable Cellular Data. This will prevent your device from using Cellular data at all.