If you have an agreement to take care of them on payment for the services, then you may enter into an agreement with properly worded draft for the purpose which may include the terms and all those conditions which may be mutually agreed.
The house property document may be pledged with you by a registered mortgaged deed for the purpose of the loan or any amount advanced to them as a collateral security.
A will also cannot be relied upon since the danger of the the revocation of the same is also imminent.
The GPA deed will entitle you to sell or alienate the property but to a third person only and not to yourself.
You may get a settlement deed registered on your father's with conditions imposed in it stating that he shall take care of them till their lifetime and can avail the property after their lifetime.