The long term goal is to do custom homes, starting with spec builds.

"Oh the customer insisted on buying the materials, it's not my fault"

As soon as you start discussing your business practice and strategies with your customers (tenants), then all bets are off and you run the risk of Fair Housing violations. 

If its a very large property, you may also want to make sure you are marketing to any wedding, family reunion, corporate meeting/team building customers. 

I'm looking for ways I can "protect" this private lender. 

They offer flexibility in design and can be customized to include a business space.Shipping container homes: While trendy, these can be challenging for mixed-use (residential/commercial) purposes due to zoning and building code issues in many areas.

While the landlord defined the dwelling unit as his single-family home; the court, after analyzing the statutory definitions of the term, the determined that the dwelling unit at issue was the separate room being rented to the tenant.[2] As a result, the landlord had converted his single unit dwelling into a multi-unit dwelling that was subject to Oakland’s Rent Adjustment Program since the individual rooms were not alienable separate from the title to any other dwelling unit.Under the court’s decision, tenants in Oakland renting rooms in a single-family home or a condominium are entitled to the protections provided by Oakland’s Rent Adjustment Program.

Tenants wont be hard to find like you mentioned but mindfulness on the protection laws in the process will be important.

Excellent customized spread sheet. 

This protects you in case the buyer defaults.Lastly, keep in mind that owner financing comes with some risk.