Single family, 2-4 unit? 

Lastly, ADUs are allowed for NON owner occupied properties, opening this strategy up for investors who could convert a SF into two units. 

I am interested in living in the basement, and renting out the upper unit on Airbnb.

That means that you can buy a property with up to four units, since this is still considered residential.

Investment Info:Small multi-family (2-4 units) buy & hold investment.

I would look at the DSCR loan as the first option.By the way, I suspect you are going to be disappointed by any refi appraisals   ADUs typically appraise poorly   JADUs can lower the property value Vs having the space as part of the primary unit.  

What's a good percentage down for a seller finance deal for multi family six unit and bigger?

If looking to buy in the U.S, there are foreign national DSCR loans for 1-4 units.

I have been trying to keep an open mind while but I see a good amount of value in long-term rentals of SFH and multi family units.

Investment Info:Large multi-family (5+ units) other investment.