Right amount for a Seller Finance Deal

[Uchenna Egwuonwu]

I’m working a deal in which the owner will agree to sell her property, a 4plex, on the terms that she‘all receive ‘x’ amount of dollars over ‘x’ amount of time. She has made it apparent that she is not interested in a lump sum amount, and would rather have payments made. 

The building has 4 units, each a 1/1 with the possibility of adding a second room to each. 2/4 units are currently occupied at $450 a month, while the market rate is $500-550.  Her monthly payment is just under 500. ~29k remaining on her mortgage balance. I estimate that repairs for both of the unoccupied units will be anywhere from 20-30k. 

What would reasonable terms for a seller finance deal be?