About all I would add is to meet with several pros, like an architect with experience in this area, lenders, environmental firm, for lead asbestos, etc, engineers to make sure the place is even sound, subs in all areas, etc...etc..

I had just completed a course on Environmental Management and planning to start a small business.

If there is an environmental issue, now is the time to know, not after you own it.Thirdly, you said many of the tenants have been there over 30 years.

Your lender may also insist upon a Phase I Environmental Assessment {but, hopefully not a Phase II}.

I work professionally as an environmental consultant in the City of SD so I'm often helping others through the (onerous) process of getting a site development permit.

Now, if there were legal issues, title matters, something more unique about the property such as an environmental issue and the agent or investor solved that problem outside the general scope of facilitating a transaction, then there is a value added as solving such issues would be incurred by another professional, an attorney, engineer, title company.Need some economics to understand the underlying justifications of "value" as well as laws of equity need to be applied.

Use the Appraisers GIS maps or BOCC maps to find environmentally sensitive or species designated areas. 

Think environmental;)

However, because the loans are commercial, the due diligence process is much longer and intensive than on a Single Family Property, requiring an appraisal, environmental study, property condition and some other case-by-case reports like Lead Based Paint, Asbestos and Radon.

Environmental Protection Agency, U.S.