IPAD 64GB should I buy

64GB internal storage really isn’t much - and is very easily to fill. So long as you are intending to “stream” Netflix or other video content for “offline” playback, 64GB might be sufficient for your stated needs.

Many contributors here would recommend that you consider the alternative 256GB model for the iPad9. 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 the iPad 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 perhaps being an absolute minimum - with more storage offering a greater level of safety.  Consider that the iPad will likely be used for the next five or more years, during which needs may develop.

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. 

Not sure what you are asking.

Yes 64GB is a pretty paltry amount of storage these days, but unless you plan on downloading a lot of 4K movies or TV shows it should not be too problematic in general.

Updates generally use around 5 to 7 GB of storage for the update process. But do not in general consume that much once the update is complete. ie. you'll won't see much change in storage usage after an update as it replaces files with new versions, and not just adds stuff without deleting anything.

If you keep on top of the iPad, you can probably make do with 64GB, and keep enough space free for updates when they come along. It's not like you can never free storage for updates, or delete contents off of the iPad to make room for the update if necessary.