Sue, if you are not planning to advertise your spare room then as far as I know, this will not count as a commercial venture and you could argue that the friends and relatives are simply contributing towards their food and accommodation costs like gas and electricity. I’m not sure that you have to register properties that are part of your actual home anyway, I think the licensing laws are aimed at separate apartments and villas. It might be worth running your ideas past a lawyer just to double check.
Regarding private health insurance, we pay €50.00 per month each (includes absolutely everything except dental care) and I have to say it is excellent and far better than anything we experienced with the UK NHS The health screening (particularly for women) and testing facilities are so good, we fear ever having to go back to the NHS. I don’t think we are allowed to name actual companies on here but we use one of the main health insurance providers in Spain.