Purchase agreement terminology

[Micki Yara]

I want to purchase a lump of rental properties (they are tied together in seller's mortgage, etc...). I want to make a contract to pay the existing mortgage off and then seller finance the rest of the balance. Because these properties are "lumped", it would enable us to seperate them, would be a better purchase arrangement for me and for the sellers. My concern is that the seller be protected (The better of the properties would be closed with the paying of the mortgage, leaving the rest of the balance, a substantial amount, with the poorer houses on the seller refinance). Clear as mud? How do I word this and what protections can I offer the seller?