The purchase price is determined by cap rate.

Then use your cap rate equation with the local cap rate and new NOI to determine the purchase price.

All that being said, I think for a scenario like this in which you were interested in putting in a offer,  I would not rely on the 50% rule to determine expenses and do a line by line item expense analysis based on historicals (from a T12 provided by owner) and the "proper" expense base.

By using "D" you can quickly determine if it meets your "D" threshold or not. 

Provide the COI to your qualifying lender and they will determine your down payment if any.

@Jared Brinn That is an important questions each investor must determine the answer to.

I had to actually go to one of their meetings to determine that it was not permitted and ended up backing out of the deal.

To determine the zoning district of your property you may use iMaps.The site then goes through the Short Term Rental Regulations. 

Hey Nathan,If there is no track record of previous income, then the Lender will typically use market rents from comparables to determine the Net Operating Income.

I found a mobile home park for sale near where I live, listed on the market, that I'm interested in determining the current and potential value of.