Be aware that you cannot add or change iPad hardware. Hardware capabilities are fixed during manufacture; Cellular capabilities cannot be retrofitted as no cellular components or antennas are fitted.
Purchase of a WiFi+Cellular model does not commit you to using Cellular features - nor are you committed to arranging a cellular airtime contract until (or if) you later choose to do so. However, purchasing a WiFi+Cellular model in preference to a WiFi Only model does preserve the flexibility to do so at a later time.
WiFi+Cellular models of iPad also include GPS capabilities. WiFi Only models have no GPS capabilities at all.
If you do choose a WiFi Only model, when paired with an iPhone, you can use the iPhone hotspot to access cellular services provide by your iPhone’s cellular connection.
More information about using Continuity to access iPhone Cellular services can be found here:
Continuity
Use Continuity to connect your Mac, iPhone, iPad, iPod touch and Apple Watch – Apple Support
SMS/MMS Messaging
How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support
WiFi Calling
Make a call with Wi-Fi Calling – Apple Support
A further alternative for a WiFi Only iPad is to obtain a portable Cellular router (also known as MiFi) from your chosen Cellular Carrier. The cost is likely to be greater than the cost differential between WiFi+Cellular and WiFi only models of iPad.