Note: Extension of the internal memory is not possible - there is no slot for a memory card! 64GB is a bad joke these days. The smartphone camera takes excellent photos in high resolution, the MP3 music collection is growing and more and more ebooks need space. If you underestimate the amount of memory you need, you won't be able to use your iPhone for long as you intended, then it will limit you and make it virtually unusable.
Only if you want to use an iPhone as a luxury phone without collecting pictures or music, 64GB will be enough in the long run.
If your interests change later and you want to store music collections or photo albums, you are very limited.
Storage in the cloud requires a reliable data connection so that you can quickly download the data you need at any time. Without a WLAN connection, this will be more expensive in the long run than the extra price for an iPhone with enough storage, as real mobile flat rates are a luxury in Germany.
You should also be aware that the NSA has access to the iCloud at all times - not permitted for business use (EU-DSGVO) and privacy protection.
Bad that you need a repair service in case of a defective battery, because you can't replace it yourself without special tools (loss of warranty).