Which is better: performing Hajj on behalf of the deceased father who had previously performed Hajj, or performing Hajj on behalf of the living mother who has not previously performed Hajj and is elderly? And is it obligatory to take the expenses of Hajj from the mother's money if she is financially capable, or is it permissible for the Muhrim (the one performing Hajj) to pay them on her behalf?

If your mother is unable to perform Hajj due to an incurable disability and she has sufficient funds, she must appoint someone to perform Hajj on her behalf. If you perform Hajj on her behalf, it is an act of filial piety, and it is not a condition that the Hajj expenses come from her own money. It is preferable that you perform Hajj on behalf of your mother because her Hajj is an obligatory duty, and because the mother's right to filial piety precedes that of the father, as evidenced by the Prophet's (peace and blessings be upon him) saying: "Your mother, then your mother, then your mother, then your father." As for seeking her satisfaction in the afterlife, this is something that cannot be achieved in this world. Continue to please her and be dutiful to her. As for your father, supplicating for him is better than performing Hajj on his behalf.