How should i make this seller finance a good deal

[Jackie Jiang]

so i found a seller willing to seller finance. His terms are 15 years @8 Interest rate. price of 255,000. The property is not in top condition and needs replacement on roof and some other things and i wanted to change one of the half baths into a full and convert one of the rooms downstairs. Base off of the terms it doesn't seem like a good deal. Its currently 4 BR and 1.5 baths and rents for 2000. If i can get it to 5 BR and 2 baths I could rent for 2,750-3,000. I expect rehab to be around 30-50k. What are some ways i can make it a good deal?