As the mortgage financing industry is constantly changing and so are certain requirements, can anyone share what the lowest down payment percentage they needed for an investment property was?

You could also see if the buyer will possibly qualify for conventional w/ a 5% down payment, which would allow them to get a new appraisal. 

I'd strongly suggest learning the laws regarding paying these individuals and how to structure the payment/agreement so you can protect yourself.  

A lender looks at it as the first mortgage payment you are going to make is your own, followed by your rentals, so there is (slightly) more risk.

He said he would do it only after receiving the payments for all the docs and the "cleaning", staging that he had paid for.

Must be: reliable, flexible, and affordable!

Also, there are no reliable comps you can count on simply because of the huge disparity between one transfer and the next along with scarcity of information sharing.A good tool that can give you an approximate value & help is the GIS system, most GIS will breakdown the land value and the improvement value.

(Constantly asking me to pay my rent early because "The electricity will get shut off if I don't make a payment today!"...

Or if you use that 50k for down payment with the rest financed with a private money lender.

When I write our loans, we have a grace period built in - typically 5-10 days - before the payment is considered late and penalties are due.