It sounds like it would be complicated. Already the property would need to be amended a lot (extension and garage conversion) to suit their needs.
If you are to be their LL and the house is to remain entirely yours, then really you should fund that. In which case (I presume) you would have to re mortgage - or at the very least pile tens of thousands of your own money in. So in order to cover that the rent would have to go to at least market value if not more to keep your figures right.
You wouldn't expect a tenant to pay for those changes. A coat of paint yes, an extension no!
If they pay to do that then really they will be making an investment into the peoperty. I'm not sure how the land lies with that.
You could probably get a contract written up along the lines of they invest £30k into the property as part rent for x amount of years, and will therefore pay only a peppercorn rent in that time. To be honest I'm not very up on the legal side of things so hopefully someone else might have a better idea.
I also think that as well meaning as everyone is, things can and do go wrong.
Illness, divorce, financial problems do happen, and while God willing they won't to you, you can never guarantee that the house wouldn't have to go.
I can see why you want to help, but you really must think it through very carefully, because it already sounds complicated, and if it sounds complicated at the idea stage - it will be very complicated in practice!