Hi Alana,
Great question! I've found that giving a seller multiple offer options is one of the best techniques to land a deal and give you an edge over other offers.
When proposing a seller finance, you can consider several levers:
1. Purchase Price
2. Down Payment Amount
3. Interest Rate (pure interest)
4. Interest Rate (with principal pay down)
5. Note Length
Normally, if a seller wants a higher price, you can ask for a longer note, lower interest, lower downpayment, etc. If a seller's sticking point isn't the price, you can offer other incentives like a high pure-interest rate for a shorter term or something like that to sweeten the deal. Take some time to make a spreadsheet with these levers and play around with them until you have two or three good offers that all make sense for both parties.
Financing the rehab can be done in a few different ways including: bringing a partner, HELOC, private money, or even credit cards (though this isn't recommended).
I'd suggest putting all your efforts into getting a good spot with your grandmother so the deal makes sense for you both right now. Once you've got the property under contract and it is a good deal, you should be able to find the money pretty easily. Right now there's more money than there is good deals around.
Good luck!