They'll give you a great deal on lumber and install. 

If something happens, can you unload the property in a hurry without losing anything?

You need to understand the risks, however, of not cash flowing and ensure you are taking a calculated risk vice just not unloading the property for emotional reasons.

The appraisal came in too low for the buyers conventional financing but they just wanted to unload it so took the hit.

It's easier to unload a single family residence than it is a condo or townhouse for the most part.  

With potentially less marketability you may have to sell at a higher cap rate in order to unload the property in the future if that is a consideration.The other consideration pertaining to liquidity is the fact that some banks simply will not finance or offer much more restrictive programs for buildings with that type of unit mix so I would talk with local, regional, and national banks to see if they have an appetite for that type of collateral.

I agree that having a third party engineer, architect, or inspector come out and verify materials is a valid idea... some shady contractors will use grade 3 lumber instead of grade 2, or simply build things wrong. 

May have an opportunity to purchase a friends home whose mother passed away and they want to unload it at $20k less than appraisal value.

They actually have a couple more properties they want to dump as well and they are open to creative financing to unload them.  

The seller may be highly motivated to unload their house for less than its actual value for any number of reasons.