Retrofit of heating cost allocators on radiators

Hello,

In the two-family house (built in 1965), where we own the apartment on the 1st floor, there is an oil heating system (about 15 years old) with water heating. The radiators do not have heat cost allocators (heat meters/evaporators) and so far the heating oil costs have simply been shared 50/50 by both parties. Now the apartment on the ground floor has been sold and we need a fairer distribution of the costs. This is also because our apartment is no longer occupied by us personally, but we want to rent it out.

My questions:

Can someone tell me approximately how much the retrofitting of evaporators on radiators costs (we have 7 radiators in the apartment + several in the basement and those in the communal rooms)?

Which type of evaporators or similar is recommended?

Is it also necessary that both parties in the house carry out the retrofitting of evaporators or is it sufficient if we have this done for our apartment?

How can consumption for water heating be recorded?

Thanks in advance for your feedback.

Thank you, Elke