I understand that notes is a business best learned by doing, but I am trying to lay a good foundation of knowledge as a starting point.To date, I have read these books:"Invest like a Bank" by Beaux Blast" Little Green Book of Note Investing" by Fred Moskowitz"Real Estate Note Investing" by Dave Van Horn"Note Investing Made Easier" by Martin Saenz"Invest in Debt" by Jimmy Napier"Profiting from Non-performing Mortgage Notes" by Amed HazelI welcome any thoughts or recommendations that the group may have.   

Similarly, the initial losses in a new business should be viewed as the "tuition" necessary to gain invaluable experience and skills in a new field.

They won't be advertising online because they are already busy.

If 2 business owners create a contract, are their signatures enough to make it a binding agreement or would getting the contract notarized be a wiser decision?

This would make it a business to business transaction. 

I am a busy doctor with some cash I have been saving for truly passive investment opportunities as I am limited with time right now.

Do this as a course of business and you may well be violating brokerage laws.

As you said it will free up some of your time and will help accelerate your business.

These loans typically require a 20% to 30% down payment depending on the scenario and DO NOT allow for owner occupancy since it’s a business purpose loan, so you’d have to rent out both units.

Which in turn will bring plenty of other investors and general business to the area.