TL, DR Solar thermal with a gas burner for peak demand. Possibly improved isolation.
First, you need exact data on how much heat you need, and when, during the year and during each day.
This should help you get an idea if the insulation of your house can be improved. Once you have the numbers, ask here for a first orientation. Without seeing the house (preferably through a thermographic camera) we will only give you a balpark estimate though.
Then, I'd lok at the following:
Solar thermal - how much collector surface can you actually mount on your house? If it's 8-storey, you will have limited roof space but maybe a south facing fasade without much shadow?
Once you have a realistic grip on how much collectors you could mount, do a conservative calculation how much thermal power this will bring you over the year (again, you want a graph of thermal power vs. time in the year) and on days representative of different periods.
See how much of your demand you can actually cover with solar thermal, decide if its worth it. Or rather: decide if it's worth to go forward with examining the possibility.
Next, look at peak heat demand and peak heat production, to get an idea how much heat storage (a huge water tank in your cellar) you will need. My gut feeling is that you will store heat for 12h to a day at the utmost, everything beyond that will be ridiculuosly large.
Look at how much of your heat demand you cannot satisfy from (stored solar), this wil stil come from your boiler. I suggest swapping for a natural gas boiler since those emit less CO2. The gas burner must be sized to supply you total heat demand, as you could have days with very bad weather and practically no sun. The burner will only be active when you need heat in but the storage tank is cold.
You will definitly need a professional to do the installation and to properly size all components. You will need high quality (well insulated) collectors to achieve high temperatures, else you'll have problems 'marrying' the solar thermal to your existing heating.