Which heating system to choose when replacing the heater after 36 years?

Ok The roof is insulated.

How about the radiators? Should they be replaced at the same time?

If they are still intact, why? You could insulate the pipes to the radiators (especially in the basement or under the basement ceiling) or insulate them better if accessible.

If they are still intact, why?

I was under the impression that the new radiators are more efficient, more economical.

Is the energy certificate the consumption-based one?

Yes

Heaters emit heat through surface area and temperature difference. If the radiator surface is small, you need higher water temperatures - that is correct. Unfortunately, I don't know the efficiency over supply temperatures for oil off the top of my head - google it, then you can answer the question yourself. However, I think it's not that dramatic.