If you research the "Garmin St. Germain Act" it goes over how that legislation allowed banks to charge variable interest rates and allow property owner to put their properties in trusts for estate planning purposes. I have yet to hear of a bank exercising the DOS. Usually a seller is highly motivated when doing sub-to deals. They are burned out landlords who are about ready to let properties go into foreclosure. You can propose it and he can say no if he wants to.
If you structure this as a lease the owner of the property will still be on the hook for repairs. What I do with my LO's is insert a clause where I'll accept responsibility for the first $250 in repairs, but according to North Carolina law the owner is still responsible for maintaining a habitable residence. So, roof leaks or HVAC would still be on the owner. This is a good exercise in deal structuring but don't spend too much time with it if the owner will not consider any of these options. I think you said he offered to finance at 8% with a 5 year call? If that's the case he doesn't sound very motivated. It sounds like he's playing hard ball. This is common in low income housing. These guys sell on "draconian" terms and wind up taking the property back in a couple of years, essentially setting up a series of buyers to fail. A lot of times one landlord will sell the same property on terms multiple times over a few decades and makes a lot of money doing it. Always be willing to walk away from any deal.
As far as taxes and insurance it depends on whether or not his mortgage payment covers the T & I in the monthly payment. Get that info from the seller.
"Master Lease", the way I have heard it explained by David Tilney is that you lease the property from him from him and take a % of the money you collect. No money no payment. He pays all repairs. This would have to be a highly motivated seller.
I think overall that multi family properties that are around the lower end require some experience to manage profitably. Single family homes in middle income have a larger margin of error. Speaking of which, do you have a screening system in place? Pulling credit, applications etc? Do you have a maintenance guy lined up? What about a trust account for the security deposits?