64 GB will be fine (!!!) as long as you are fine with iPadOS managing the storage (interesting read: Why am I having to re-download iCloud files on my iPad?).
My opinion would be to get either the iPad Air 64 GB or the iPad Pro 128 GB. If the iPad Air started with 128 GB, I would say: definitely get the Air given its entry price! 
There are other considerations apart from the storage capacity. Do you prefer Touch ID (Air) or Face ID (Pro)?
A good read also:
For me, the iPad Pro 128 GB would be the answer.
But, you could also prefer the Air:
It is not easy…