Apple just announced the new iPhone SE for $399! However for just $50 it doubles the storage space from 64GB to 128GB and that's what I would recommend. The iPhone SE can shoot 4K video at 30FPS which means each minute takes up 170MB (1 GB = 1000MB) so 5 minutes of video takes up 1GB! Also system storage for iOS 13 takes about 7GB of space so your 64GB iPhone is more like a 57/58GB phone and then apps, music and movies take up space. Oh and attachments in iMessage.
Anyways hope this helps!