You will also want to make sure that the property is zoned properly. 

Contractors are SUPPOSED to know what is required through L&I and they should obtain the proper permits.

As Remington said I would recommend talking to an attorney to get proper advice about this though.  

We like the factors you mention...the sales price range is more affordable than in "SF proper."

And that's not even getting into whether the "pro" would properly diagnose the problem. 

Those that don't work with borrowers that have rentals, may not have the knowledge of how to set the loan up correctly and properly count all income from all rentals. 

As such, it is necessary to learn how to properly identify rehab costs and there are literally hundreds of threads on here on that topic, many of which I have gone into great detail on as have others.As an example, two identical houses on same street, model matches.

What is the proper process to successfully partner with others? 

You can literally buy your wealth, and constantly be lowering your debt ratio with proper planning and execution. 

The sellers accepted the offer and signed it, but after 2 or 3 times of asking for the info and fixing the disclosure sheet, we still did not get any information or the disclosure sheet filled out properly.