Hello,
I would also consider the heat meter as the best solution. This provides a measured value that can be used for billing, and the problem of the instantaneous water heater with rising electricity costs is thus eliminated.
Have a heat demand calculation made. This shows the heating load as a criterion for the size of the heating system. It can also be used to determine the required sizes of the other heating system components.
A rough estimate certainly helps to get a feel for it, but a calculation method should be used as the basis for the final selection.
Beste Grüße, Erik