Our advice has always been to purchase the latest and largest iPad Pro model, even if it means having to sacrifice on storage space or cellular connectivity, as the latest model will give you the most hardware longevity and screen real estate.
Currently, the best iPad to use with our apps is the 2020 iPad Pro 12.9" (4th generation). The larger screen size and fast processor has definite advantages when working with a lot of documents and videos. For storage space, 256GB is ample for most situations, but if you plan on storing a lot of videos on your iPad you may want to get the 512GB. You can compare the different models here: https://www.apple.com/ipad/compare/
If you choose an iPad Pro it is compatible with the Magic Keyboard (https://www.apple.com/ipad-keyboards/) which is a fabulous accessory with a built in trackpad. An Apple Pencil (https://www.apple.com/apple-pencil/) is not required as you don’t draw in our apps, but it does offer a preciseness when doing any kind of annotations, such as highlights and callouts.