Some say its because DF/SAFE are designed primary for residential mortgages and not MHPs. 

1.Ask for a copy of their standard seller side and buyer side closing documents2.Ask if they close any non-traditional type transactions you intend to use.3.Ask for a copy of their standard fees, and if title policy rates are not set by a governing body4.Ask about insurance premium rates.5.I would ask them if they are investor friendly.6.If they double close and if what do they, ask what do they charge for the 2nd close?

The house will not qualify for a traditional loan in its current condition. 

What are the benefits to this type of system vs. one with traditional internet? 

I own several rentals and rent, out of choice mostly.I’ll probably have 20 rentals before I buy a primary residence

However, I have seen with new construction, that side by side units require much less red tape to build new as opposed to a traditional "decker" style multi.   

Im in a similar situation with a municipality, but if you are able to cover health insurance through the service, that makes the transition out of the traditional W-2 much more feasible and also covers all of your bases.  

I'm sure they have a policy on short term rentals.If you mean "legally" based on the terms of your loan (i.e. one for a primary residence), I would imagine that would be fine as long as you are living there.

Unfortunately, my only real estate experience to date is my primary residence.

The rental calculator here on BP uses a traditional mortgage, and obviously seller financing isn't quite the same.