Negotiating a Private Sale for Purposes of AirBnB

[Robert Newcombe]

Here's an overview of my situation:

-Neighbour of my mother-in-law is looking to sell privately, his children/family aren't interested in his house.

-The house in question is on a beautiful oceanfront property with private beach (just outside Halifax, Nova Scotia) but is an old house (80+ years).

-He's had some interest from others to sell to them. They would buy the property, tear the house down, and build a million dollar home. We would keep the house as is for purposes of STR since the structure is solid, it has a nice cottage feel, and also has 4 bedrooms.

-Fortunately, he wants to sell to my wife and I first with the close connection but is asking a lot (since realtors have told him he can get a lot - $550-$575k for a house that many would tear down). Unfortunately(?), there's no great comps for that type of house on that type of property.

My wife and I would love to eventually retire there and it's such a good opportunity for a short-term/vacation rental for years/decades. Any advice for negotiating on a private sale? Would getting the lawyers involved to negotiate for us be a good play? Is there a target average daily rental rate that we should be aiming for to make a selling price like $550k work? 

Thanks in advance,

Robert