TL, DR: How do I make money on a deal where I can't figure out the after repair value (ARV) on a property? Need suggestions on how to value it.  

I am working with an individual seller who is willing to sell his doublewide mobile home, 40 x 40 commercial building and 3.5 acres of land to me. They are flexible on price. The one problem is this: I have no idea how to actually value the land and property. 

The tax assessor's market value for the property and land is $87,300, split between $18,300 for the land and $69,000 for the improvements. Some automated services I use to value the property value it between $112,000 and $130,000. I have seen one house (not mobile home) within a quarter mile (1/4) radius in the last six months sell for up to $300,000 but most of them are around $150,000. 

The owners are asking $80,000. I have no idea if this is a fair or overpriced because I'm not entirely sure what the back end after repair value is. The only thing I can think of doing here is to enter into a contract for deed for as low a price as possible (e.g. $50,000) for as long as possible. While in the contract for deed, I would list the property to sell it. Once again though, the problem is I don't know how to determine an after repair value which is how I would back into what I can pay them. 

Any suggestion you might have on valuing or approaching this situation would be greatly appreciated!

Additional keywords: valuation, purchase land, lots, atlanta, seller financing, land contract, wholesale