If I want additional marketing for specific vacant units, how would we arrange that?

You can collect bids there from cleaners and arrange meetings with them.

Just curious what is the typical arrangement when buying on seller finance?

I would very much like to do that, how could we arrange that?

Something like à viager arrangement.

@Charles Granja I can imagine its very difficult to find owners with assumable VA loans for 3-4 unit properties that want that type of arrangement as opposed to just selling and freeing up their entitlement, but I'm sure its possible!

I am also leaning toward arranging for a slab to be poured. 

Are there any verbal agreements or arrangements between the current Landlord and Tenant?

Without pre-arranged terms, its not a toss up whether the sellers "care" about your situation.Like I said, its really a matter of how bad do you want it. 

I am researching the market and will start diving deep into cleaning arrangements, repair folks, what types of properties bring the biggest return (size, more private vs more crowded cabins, closer to or further from town, hot tubs, views, etc...)