ᐅ Sizing of Solar Thermal Systems

We are planning a solar thermal system with 5 sq m (54 sq ft) of collector area installed on the roof, combined with a 300 L (79 gallons) buffer tank. The system will be used for domestic hot water preparation. My questions are:
a) Is it worth increasing the storage volume to use it also for heating? The storage volume is not yet fixed.
b) What about circulation with regard to legionella risk? Which type of buffer tank is recommended in this case?

The collector area is fixed (I will not go into further details on this). Under these conditions, what is the most effective solution to consider, especially regarding safety aspects such as legionella prevention?

We also have the 300-liter (80-gallon) tank. There should be no risk of Legionella since the water temperature is well above 60°C (140°F). Using heating support is pointless. In summer, you don’t heat, and in winter, solar output is minimal. Just skip it; it’s a waste of money.

Is the buffer tank equipped with corrugated pipe or a tank-in-tank design? Which is better? What about the risk of the fluid in the heating circuit overheating if no water is drawn? I have heard that many systems get damaged because of this.

I don’t know any of that, they installed it, I’m just using it.