Wi-Fi vs Wi-Fi with Cellular

Which do I need if I want Maps (Google or Apple) to work as a GPS?

While all models of iPhone have GPS capabilities to find precise location, only WiFi+Cellular models of iPad have GPS - this being a function of the iPad Cellular chipset. By contrast, WiFi Only models of iPad have no GPS capabilities with which iPad Location Services can derive location. This fundamental difference may significantly impact your ability to use mapping Apps - in particular if precise location or real-time positioning (e.g., a moving map) is required.

WiFi Only models can only “infer” their location from a database lookup of neighbouring WiFi networks for which the geographic location is known, or the geographic location of your public IP address. For static applications in urban locations, where the location of IP public IP addresses and WiFi networks are known, an “inferred” location can be relatively accurate. However, in rural locations, or when using a Cellular/Satellite/VPN connection, location accuracy of an inferred location [if it works at all] may be very inaccurate.

All this has significant implications for non-static mapping applications. If you need to have moving maps, or have future flexibility to do so, you’ll need to consider a WiFi+Cellular model of iPad. Choosing a cellular model does not commit you to adding a cellular service plan for the iPad, but ensures that you have immediate access to GPS/GNSS positioning services.

Credits -- https://discussions.apple.com/thread/254581504

If I chose Wi-Fi with Cellular, must I purchase a data plan and Sim card for the device?

No.