I'd recommend an energy audit of the house first: in winter, how much energy comes in as heating, how much comes out as losses. And in summer, how much unwanted energy sneaks in, making the house too hot.
Only when you've done that and put numbers can you make informed choices, otherwise it's all guesswork.
This heavily depends on how the house is constructed, where it is located, and its orientation. I know some places in Portugal are pretty hot, but others get rather chilly winters.
As for solar heating, I did a study on the house, but it turned out installing more insulation was a better option. Basically, if you want solar heating, then you need to store heat from sunny days and keep it over the short term to heat during the night, and over the longer term for overcast days where the solar system won't work. This storage can use quite a lot of water volume, and it requires oversizing the solar collectors, so one day of sun will recharge the storage enough to last for 1-2 overcast days. This means diminishing returns, decisions to make, and an auxiliary system for long overcast periods.
If your house was built in a time where energy was plentiful and cheap, and in a climate where a lack of insulation wouldn't result in too much expense, it is likely its insulation is going to be terrible. This means this should be investigated first, as it would be the low hanging fruit, with the highest potential for improvement.
Basically, if you live in a sunny place where winters seldom go below 0°C, with a good amount of insulation and south-facing windows, you should barely need any heating.
Note you don't need solar collectors for solar heating. My house faces south, and it has large windows. So, I installed a large awning on top, with vine growing on it. The leaves provide shade in the summer, so no direct sunlight gets into the house. Then in autumn they fall off, and the sun is lower on the horizon, so I get a lot of sun inside the house in the winter.
