Raspberry Pis with WiFi are very useful for building prototypes that connect to services on the Internet.
In case you need a Raspberry Pi with in-built WiFi, you can get one of these:
An operating system is needed for running your Raspberry Pi and Raspbian Stretch Lite is an ideal choice for server based applications.
Although Raspbian Stretch Lite does not have a graphical user interface, it is not difficult to get it to connect to a WiFi hotspot.
If you wish to use your iPhone as a WiFi hotspot for your Raspberry Pi prototype, read this post to find out how to connect Raspbian Stretch Lite to your iPhone hotspot.