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

Using percentages as a guide to success tells you nothing of value. 

The percentage depends on factors like project complexity, timeline, and contractor availability.

What percentage do you typically allocate to this with your properties, or when you are assessing a property?  

My personal holding company would own the equity position for my percentage in this syndication.

There are tons of filters you can use including out of town owners, single family, multi fam, equity percentage…etc. https://www.melissa.com

A large percentage of these folks will apply elsewhere if that is clear in the listing. - Even with this caveat, some tenants will attempt to go to the property.

If you go fully custom, it will be higher.As for using land as collateral for a construction loan, it’s definitely possible, but lenders usually want to see a decent percentage of equity in the land.

In your proposed case you would be compounding this negative position by taking away the best source of financing for a large percentage of potential buyers.  

As a percentage closing cost as percentage goes down as value increases but we will 15X to account for closing costsNo cash flow per OP.2% market appreciation equated to 30% + 15% = 45% 3% market appreciation equates to 45% + 15% = 60%4% market appreciation equates to 60% + 15% = 75%Recognize in virtually all markets the cash flow increases with hold length especially if a fixed rate loan.