Is the seller wanting to stay in the house? Are they willing to give you a deed? This just seems fraught with potential problems, mostly around the seller trying to go back on the deal after you've invested the time and money.
I think the most straightforward way would be to structure this a subject to deal with an equity share to the owner when the property sells. You buy the property for its current outstanding loan balance and take over the loan. You pay to fix it up and make the payments. When it sells, the existing loan is paid off, and you get paid back for both the payments you've made and the investment you've made into the rehab. Whatever's left gets split between you and the seller.
Even if you let her make the payments, I think you should still structure the deal this way. If she makes the payments, she gets paid back for those. Your net is the same, but you're not out of pocket for the payments.
Now, she may balk at having you own the property while she makes the payments. So, maybe you could create an LLC. She contributes the encumbered property and makes the payments. You contiribute your time and money for the fixup. After sale, the property gets paid off, and you each get back what you put in. Then, the LLC is wound up and you split the remaining cash 50/50 (or whatever.)
Jon