Are you also asking what the "blue sky" part of the deal is worth? Aside from the actual realestate? In my opinion owners usually think it is worth more than it is. You should really get the whole business apraised not just the land, fixtures etc.
They maybe the nicest folks in the world. But business is business, Don't count on friendship, in any Business deal.
Ask yourself this, If you buy it and the place fails, or you find major structural problems, Will they give your money back? That would be very rare indeed in my opinion. Much more likely the responce would be "sorry about that" And good luck with it, Don't call us again.