Specialize in properties that have environmental issues3.

Then you have your property survey, phase 1 environmental and appraisal

The way around that is to have your contract have a tight clause that only allows termination (not for "any or no reason") if they find a mechanical, insect, electrical, plumbing, environmental or structural issue that the seller is not willing to remediate. 

Risk can be things like low quality tenants, crime (violent, drug, and property), environmental, political, economical, etc...With that in mind, there is not bare minimum ratio to search for.

Are there any payments upfront (environmental study fees or something)?

High interest rates are the environmental ingredient you need for good seller financing.

The environmental elements I endure in the northern rocky mountains of Montana are my biggest challenges.

Does it have environmental contamination or title issues?

I knew this property and the environmental would be a tricky deal and I have limited experience. 

Scott Curriden of The Scripps Research Institute's Environmental Health and Safety Department.