Strategy for Seller Carry Back

Hi, I’m thinking of trying to purchase aan apartment complex that was recently renovated. 30% occupied ; 72 units totaled(the seller will start a drive for 100%) however, the list price is $1.4 Million. I just received an email that he was willing to do some carryback. My question is to purchase this property with no money down how can I create terms that would allow this. How much carryback would I have to propose and it’s term?