Problem is there are to many investment types.  

Do you have a preferred type of property and price range you're looking for in general (knowing the costs can be different based on they market)?

Post Dodd Frank sellers have continued the same bad practice of selling houses to occupants with financing at selling prices that are way high, can't be justified via any type of 3rd party valuation (appraisal, zestimate, tax value, even recent sales in the same area).   

Typically, the adjustments I see on appraisals for basements are more flat rate adjustments. 10k is usually the number I've seen.What stinks is that there are a couple of model types that have below grade space where the builder, I'm sure, was trying to lower the taxes by having it below grade so the county wouldn't count that space in the assessment.

What made you interested in investing in this type of deal?

This Wikipedia page gives different wall types and how well they block sound.  

* What types of evidence would we need?

What type of properties are you looking to acquire, SF, 2-4 MF, or 5+ MF?

Do you have any types of properties for rental (SF, 2-4 MF, or 5+ MF)?