Looking for property purchase advice

[Geoff S.]

I've found a 3 family property in a decent part of a working class town. It is in excellent shape and owner occupied by an eighty year old couple. They want to sell and eventually move on. They hope to move into a a retirement community after the sale and after finding an acceptable place. This couple has maintained and managed this place with good quality material, equipment, workmanship and as good landlords. All three units have separate utilities with other two tenants paying their own way with electric and heat as well as with market rents.

The numbers all work and I will be making them an offer. What I am not sure of is how to make the offer with the possibility of them staying on for some time say six months or so. I would love for them to stay as long as possible since they know the property and the tenants. Should I just make the offer as though they were just any other tenants in the building and secure a month to month lease or make their stay part of the deal. Does buying an option on the property and closing when they decide to move make sense ? Maybe I'm making more of this than I need too, any advice?