I want to accept only money orders, checks, direct deposit and or electronic payment through Zelle.

In my opinion the best scenario for you would be as follows:1)  5 yr lease with a 3% rent increase every year on the year anniversary.2) Require proof of ability to fund the build out project. ( Letter from Bank, lender )Require a one month rent or more security deposit at lease signing. 3 ) You should have building insurance ( build out/ construction insurance ) started at lease signing.

If you used a realtor, the deposit should be held by their realtor and wouldn't have been released to the seller.

The couple had been living with their Aunt and Uncle until they had enough money saved up for a security deposit.

You'd have no security deposit to deduct against.

I have a few investors reaching out to me to do business with, but I don’t personally know them and I’m naturally cautious because I want to make sure any deals I do are clean and that I’ll receive my initial deposit and profit in return.

@Jim Truman as @Aaron K. mentioned, look at the Schedule E and it’s pretty easy to setup an Excel sheet that matches the Schedule E.

Is it an option fee that is nonrefundable or a true Earnest Money deposit? 

As such, it is necessary to learn how to properly identify rehab costs and there are literally hundreds of threads on here on that topic, many of which I have gone into great detail on as have others.As an example, two identical houses on same street, model matches.

I would ask the builder to put down a larger deposit in light of the transient, unnumbered, changing, or unverified nature of the tenants (if legal)2.