All iPad models have broadly the same core capabilities. Models essentially differ in their ultimate performance and physical attributes. iPad Pro models use FaceID for biometric user-authentication, while others use TouchID.
You can compare iPad models here:
iPad - Compare Models - Apple
The iPad A16 is the current entry-level iPad. The iPads Pro M4 are the opposite end of the scale - top of the range, with cutting edge capabilities and hardware. Between these two extremes is the iPad Air M3, this model offering a mid-range high performance option. The iPad mini A17 Pro is perhaps an entirely different proposition - offering a smaller form factor and other differences in hardware and ultimate capability.
All current models of iPad support Apple Pencil. Ensure that you choose the correct Apple Pencil for your chosen iPad - as they are not interchangeable. Here is the definitive compatibility list:
Apple Pencil compatibility - Apple Support
Also perhaps consider that the iPad Pro M4 models are the only iPads to offer an OLED display, this being the most power-efficient and offers greatest contrast and brightness of any model iPad - and also includes reference modes for professional colour grading. Also note that the current iPad Pro models with 1TB/2TB storage options have 16GB RAM, whereas the others have 8GB RAM.
Whichever iPad model you choose, you would be well advised to purchase as much internal storage as your budget might reasonably allow - as internal storage (and other hardware) cannot be changed, upgraded or extended. The available internal storage is fixed for the entire life of the device. If you run out of internal storage for your documents, photos and other data, you will forever struggle and become a source of frustration. It is far better to have more storage than you need, than to need more storage than you have.
A good rule of thumb is to quantify how much data storage you will need - add a healthy margin - then double it. If you are unable to quantify your data storage needs, it becomes more difficult. Whilst I and others cannot make definitive recommendations for your personal needs, objectively I would council against purchase of any iPad with any less than 256GB storage, 512GB and higher having a greater margin for growth.
Within the Apple Support Communities we see regular tales of woe and disappointment when storage requirements are exceeded, necessitating replacement of the device; by contrast, we never see complaint of having too much available storage. It is relatively easy to fill limited internal storage space; perhaps consider higher tiers of storage as providing room for growth as your needs develop or mature - potentially extending the useful usable life of the device.
Also consider the functional differences between WiFi Only and WiFi+Cellular models of iPad. Aside from the obvious Cellular functionality, only WiFi+Cellular models include GNSS/GPS location capabilities - which may impact any Apps or features that require accurate and consistent location data, such as Mapping.
Purchase of a WiFi+Cellular model provides access to precise location services, but does not commit you to using Cellular connections. WiFi Only models of iPad have no Cellular or GNSS/GPS functions at all.
