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.

(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. 

Is a down payment typical, or does that really just depend on the deal?

:1)Find an investor who would double as a source of money for down payment and a cosigner2)Use an FHA loan (3.5% down if I live in a unit) to buy a quad-plex3)Use the 203k aspect of the loan to improve the quad-plex and force appreciation in the property4)Refinance in a year or two to a regular loan and pull equity out to either pay off my outside investor or use equity for another down payment on another quad-plex5)Repeat With this plan, I believe I will be able to cash flow from day one because I will be able to rent out three units to college students around campus who are willing to pay higher rents than most.