Preamble
All current iPad models are capable of running the current iPadOS Operating System - as of this week, this being iPadOS 17. 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 iPad9 is the current entry-level iPad. The iPads Pro are the opposite end of the scale - top of the range, with cutting edge capabilities and hardware. Between these two extremes is the iPad Air5, this model offering a mid-range high performance option. The new iPad10 is cheaper than the iPad Air - and differs in position of the front-facing camera. The iPad mini6 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 either the first or second generation Apple Pencil. For handwritten notes and sketches, this will be relevant. 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
iPad models that support the second generation Apple Pencil are paired and charged using the magnetic charging dock on the edge of the iPad. This greatly simplifies charging of the Pencil - as the Pencil is kept optimally charged whenever it is not being actively used. The second generation Pencil has some additional features - aiding drawing capabilities.
Also perhaps consider that the iPad Pro 12.9" model is the only iPad to use the miniLED backlit XDR display. This display is not only the largest in the iPad range, but is also 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 "only" 8GB RAM (this still being greater than other iPad models).
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.
Specific responses to your questions
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