We just purchased a 12 plex for $675,000 with $7700+- rent totals. We financed with a bank so I can't speak to the owner financing piece/ lien on current property.
Our monthly expenses: are $3000 mortgage, $700 taxes, $350 insurance $650+- water and electric. $4700 in expenses, $7700 in rent sounds good--- but we have been negative--- some months our expenses have been more like $12,000!! We knew the first year would be tough but keep you eyes open!
Since you already have properties I'm sure you have done due diligence. With that being said, do your due diligence!!!
This was our first purchase and we are thrilled about it. However 6 months in we are no where close to a positive cash flow. We will be in another year, I hope. We knew we had major expenses (roofs, decks etc.) but we should have negotiated repairs better IMO. Don't feel like you can't negotiate. The Seller obviously wants to Sell, just don't get in over your head!
You have more experience than we do, just wanted to share since the numbers are somewhat similar. Negotiate for yourself and good luck!!