This sounds more like an ADU conversion, so I would start there.

For example, an approximate price per square foot for light, moderate or heavy rehab?

Successful people love to share their experiences and I find that particularly in real Estate people are willing to take time to share, educate, mentor, or motivate people who have an interest.

Wouldn't be surprised if an OM comes out from this corner. 

After I sent an LOI, the seller changes his story, and tried to push me into the deal, and keep telling me that as "men of honor" we don't need to put any of the changes to the deal in writing. 

You should anticipate the following fees: Loan Fees: Appraisal FeeLoan Origination FeeUnderwriting Fee Processing Fee Document Prepration Fee Credit Report Fee Closing Fees: Title Insurance Fee Attorney Fees Notary Fees Escrow Fees Wiring Fees Maps/Special Report Fees Prepaid Items: Usually the insurance premium is paid at closing Prepaid Taxes and Insurance for escrow account (If taxes and insurance are impounded) Interest (depending on the day you close) I would suggest getting an official Loan Estimate from your lender.

Will talk to an attorney

Personally that is what I wanted, an active mentor. 

Just wanted to give an update, my buyer rented both units within 2 weeks of purchase and loves this property.

generally speaking most of the time you have to hook to Sewer if its in the city .. not many cities allow septic systems.. on that map your septic drain field does not look near big enough and they will mandate an area that is 100% same size for back up when the original system may fail.