I have the following questions.
1) Realistic rent? Rent of 3500 in La Mesa is quite a lot, where one expects a recent remodel and/or great amenities. I'm not sure if a house in need of TLC can command that, let alone one built more than 100 years ago.
2) Seller finance: The terms the owner offers is 25% down at 6% interest with a balloon at 5 years (presumably 30 year amortization). If you have good credit, that's about the same terms as what you get from a good lender these days. So why seller finance?
3) What is the deal about "discounting the purchase for rent"???? Is the owner going to live there for another 5 years and you are going to lease the property back to her? By "discounting the purchase" it is a form of prepaying that rent and then she expects you to sign a lease to rent the unit for her for $0? If so you want to read the details of AB1482. If AB1482 applies, your rent increase is limited to 5%+inflation (i.e. $0 for you if you start from $0) and you are not allowed to terminate a lease except for just cause(tenant issues or you are moving in or major renovation). There will also be an eviction issue if she decides to horde the place. And is she going to live in the main house or the granny flat? Why isn't she paying the rent she claims to bring (3500 for main or 1900 for granny flat), and factoring in the annual rent appreciation?
4) I assume you have done a thorough inspection for roof, foundation, drain line, and termite damages so that the purchase price had properly factored these in?