Am I right in thinking you want a biomass boiler as a ‘back burner’ which would be pretty hard to get at? Would this not be a nightmare to clean/service? How are you getting your wood chip or pellets into the boiler?
Would you need a buffer vessel for your solar panels assuming they are for the hot water? Have you considered any plant space?
Ground source heat pumps are pretty expensive due to the civil work involved unless your digging the place up anyway.
Not sure how it effects domestic stuff but check out the renewable heat incentive.