Since it is not clear from the questions and specifications: After the initial costs for the scale, are there any further costs for an app plan or other?
We use the scale without any plan costs. But we experienced that the scale does not deal well with the requirement that several people connect to their mobile.
No, the scale can be used in the described scope of functions without additional costs. With a 'Withings+' plan, it is possible to record additional data, but this is not mandatory.