Formulating seller finance offer

[Robert Morneweck]

Trying to find my first deal. There is a 2/1, 880 sq.ft. "cottage" in the path of progress. Listed on Zillow for 65,000. Current occupant pays $600, thru February (<1%) and the place "needs some work". I believe the rent could be higher due to location(2 beds in area go for well over $900) and about $10,000 worth or work. On to my question...seller states it needs to be a cash deal, is there a rule of thumb on how much higher I should offer if I want to approach seller with the idea of seller financing?