But as Chris mentioned, buyer has to own the property, not a contract for deed or something like that.

I got the building under contract and then figured out financing.

I took out a loan with one of my friends, wrote up a loan agreement contract that we both signed and paid him the interest on a property early last year. 

We are under contract on a property right now and negotiated $40k in credits to use to buy our rate down.

Quote from @Aaron Zimmerman: I would look for other ways to legally reduce income such as cost segregation studies.

But, if steady long term cashflow is a goal, with no management, one can always sell farm land on a low down private contract.

Also, what's the name of this type of contract where it's just between you and the seller?

Are you actually building the ADU or contracting out? 

I don't necessarily need the standard features of pm software, such as rent collection, bookkeeping and maintenance requests, though that could help; but do need a clearinghouse of important information, such as leases, legal docs and other information; maintenance vendors I use; maintenance history; and some kind of tracking system, where I can record the various current projects I'm working on within the business.

This also leads owners to ASSUME simpler is better when it comes to management contracts.