It requires significant work, and I'm concerned that some additions made by the previous owner may not comply with building codes.

The owner is asking 600k but I'm not sure his valuation is accurate.

Go to Etsy and search for "rental property tracker," and you will find hundreds of nice spreadsheets to track 10-20 rentals, usually for under $10.Software has extremely helpful features like online payments, marketing syndication (click a button, and your property is advertised on multiple sites), electronic document review/signing, maintenance tracking, and owner reports.

You may already know this (you should know this), the JADU requires owner occupancy on the property to be rented as an individual unit.  

However, there are some bridge loans that help owners capture some of their equity to buy a replacement property, while listing their current home for sale.

An "owner's title policy" is purchased (typically) by the seller for benefit of the buyer. 

To address this, we verify the property owner per government records and use this information to vet imposters.Anything that seems suspicious will lead to us requesting an acceptable Letter of Explanation to address.Analyze IncomeMany landlords require a month of paystubs from applicants and just use these to calculate a monthly qualifying income.

Many are private owners just subsidizing their mortgage or they would not be able to afford the house without renting it out much of the season.

Or ask the third owner if they would also consider selling, so the whole property is sold.Whether a partnership or LLC, follow the Operating agreement. 

Does the owner use property management now or manage themselves?