He doesn't even know why his attorney structured it in that manner.

If the homeowner doesn’t comply within a specified time frame, the city may step in to demolish the unsafe structure and bill the owner for the costs.

Have you looked into a potential different structure with instead of a seller note, structure it through equity in an LLC partnership? 

However I'm wondering what is a good roi and structure. 

I'm currently spending time learning as much as I can from various resources including books here on BP, accumulating capital and structuring my strategy. 

I have read many articles on the types of real estate investment strategies like wholesaling, buy and flip (and buy and hold), structuring deals for no money down seller financing and even investing in notes! 

Were you successful in setting up this structure and keeping the loan without triggering a due on sale clause?

Yes, you can; this strategy is called pooling and it also adds a tremendous tax advantage You'll just need to make sure the entity pulling and disbursing the capital is structured correctly for its maximum benefit  

These loans come from professional private lenders, are more structured, and have higher interest rates (8% to 15%) with shorter terms (6 months to 3 years).

I am just curious how most structure their payment process when receiving and paying rental payments, vender payments, etc.