I pad

The current entry-level iPad9 is the cheapest option from the available range - and being a current model capable of running iPadOS16, will likely be currently available amongst the models with longest update support. Older models are perhaps more risky in terms of technical specification, being less capable of running some Apps with adequate performance.

Being the cheapest of the current model range, principally designed for educational environments, the iPad9 is also one of the more physically robust. That said, always consider a good protective case.

With limited budget, perhaps consider a reconditioned model from Apple - rather than risking a “used” device from other sources; previously owned devices from other sources are potential minefield. Reconditioned models sourced from Apple or an Apple Authorised Retailer come with a warranty.

Whatever model of iPad you choose, you would be well advised to purchase as much internal storage as your budget might allow - as internal storage 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 cannot make definitive recommendations, personally I would council against purchase of any iPad with any less than 256GB storage - 128GB being an absolute minimum.

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 also the functional differences between WiFi Only and WiFi+Cellular models of iPad. Only WiFi+Cellular models include GPS capabilities - which may impact any Apps or features that require accurate and consistent location data, such as Mapping. Purchase of a WiFi+Cellular model does not commit you to using Cellular connections, but provides this flexibility should it be needed.  WiFi Only models of iPad have no Cellular or GPS functions at all.

I hope this information and insight proves to be helpful to you.

As Lotus Pilot suggested, purchase the base model 2021 iPad 9th gen.

Get the largest storage capacity you can afford.

FYI

A 64 GBs storage iPad starts out with approx. 60 GBs.,after initial internal flash drive formatting.

Minus approx. 6.5 GBs for iOS/iPadOS leaves between 53-54 GBs.

Minus 3-6 GBs, for iOS/iPadOS system operational reserve, that MUST BE MAINTAINED AT ALL TIMES, to ensure/insure normal, expected operation of iOS/iPad OS, leaves approx. between 48-50 GBs of actual, usable data storage space.

256 GBs storage iPad starts out with approx. 238.5GBs.,after initial internal flash drive formatting.

Minus approx. 6.5 GBs for iOS/iPadOS leaves 231.0 GBs.

Minus 3-6 GBs, for iOS/iPadOS system operational reserve, that MUST BE MAINTAINED AT ALL TIMES, to ensure/insure normal, expected operation of iOS/iPad OS, leaves approx. between 224-228 GBs of actual, usable data storage space.

The 2021 iPad 9th gen iPad should get, at least, 6 to 7 years of full support from Apple and third party app developers.

And add another 2 to 3 additional years of third party app support AFTER Apple ends its full support for their mobile devices.

Best of Luck to You!

You are very welcome.