Uwe, in all the rent-to-own type of deals your hope is that you will eventualy own the property. It's a sort of owner financing.
With a master lease, you will never be anything but the renter. You rent the entire building, and then it is your responsibility to take care of the whole thing. You get the tenants for the individual units and you keep any rent that they pay.
Like I said above, its more common to have a master lease on something like a shopping center. You rent the entire shopping center, all the buildings, all the parking lots, and then you find your own tenants for the individual stores.
With a master lease, the owner of the building is not selling. You aren't buying.