Hello,
Nice in theory, but ...
I do not believe that a Bank will lend you money based on 75% of the house, so you would have to get your mother to agree to pledge her remaining 25% share as security, to facilitate the Bank obtaining a mortgage over the entire property. Even then, the Bank may be reluctant to give a long term mortgage given your mother's age.
What happens if something changes in a few years time - say for example if you are no longer able to pay the mortgage (and property values have dropped so the entire house value is needed to clear the debt), or if you need to move to another location and want to sell the house, but your mother does not etc ? Also, what if your mother needs the remaining equity and you are not in a position to buy her out when she needs the funds ?
Family members may all mean well, but sadly the risk of a dispute breaking out about an arrangement like this at some point in the future is probably high.
You are quite right to mention getting contracts signed up and this paperwork would need to be perfect. Furthermore, depending on your mother's age, is there the possibility that someone could claim that she is not in a fit state to be entering into such an arrangement ? ..... I would be getting some legal advice and if I was going ahead with this, I think I would want all of her other children to be signing the documentation as witnesses to it, or swearing affidavits to confirm that they knew about the arrangement and were supportive of it etc.
My initial reaction is that there is no particular tax concern for either party given it's a principal private residence, but that is advice you need to get from a suitable professional.