The best advice is to purchase as much on-device storage as you can afford - as iPad internal storage cannot be altered or updated.
Far better to have more device storage than you need - than to later discover that you need more storage than you have. You can never have, or see complains of having, too much storage - whereas you will see many tales of woe from those that sought false economy in choosing a device with storage inadequate for their needs. Having more storage than you perhaps anticipate provides room to grow as your needs mature.
Dependent upon your intended use, 256GB may be inadequate. Consider that tasks can only be completed on data stored (or cached) locally on the iPad. Whilst reliance on Cloud storage is useful for backup purposes, it is of no use for creation, manipulation or modification of data. Consider also that up/download of large files over a broadband connection may be time consuming - or “expensive” in terms of network utilisation. Over a Cellular network link, if used, data exchange often comes with a recurrent financial cost.
A general computing “rule-of-thumb” is to think carefully and quantify your expected storage needs - add a healthy margin - then double it.
My recommendation - and that of many that participate within these User Communities - is to purchase as much internal storage as you budget will allow. Personally, I would recommend that a 512GB would be a much “safer” option, noting that some models of iPad Pro are available with 1TB of local storage. 512GB is, IMHO, a “good” specification that allows sufficient storage; 1TB devices may be overkill, but needed by a few power users.
I hope this information and guidance proves to be helpful in to you.