But the downside of spending the cash and then actually needing it for operations at a later date is too high.You are in a very difficult position and I would recommend that you keep communication as clear and unfiltered as possible between the two parties and keep your personal interest out of your communication decision path.

Perhaps a nice 1950's built bungalow.Always hire a 3rd party property inspector to give you an unbiased feel for the home.

I got tired of having two middlemen between me and the other party.

For the most part, I have used cash out refinances and home equity lines of credit to get the cash together to continue investing. 

If you make your monthly payment through a third party servicing company that then disburses to the seller it is possible they do report (some but not all do), but if you pay directly to the seller then that is unlikely.

I point out either it is open or part is not.

The short answer is:  your question is not scalable and requires buyer/borrowre qualifying knowledge and steps and thought not requuired for 1 per year depending on yiur states Safe Act's limits, its suggested that you use a 3rd party Licensed Mortgage loan Originator. 

We have a strong relationship with the seller and both parties are happy.

You can use other 3rd party custodian services.

After all, you two are the true principal parties in the deal.