Plus you'll most likely make more money in home value growth compared to a multi family. 

You've earned $132,000 more by splitting your money and leveraging it.

I will add that to your question of 10% or 20%, you need to ask yourself whether you need the cashflow or not. 10% down would leave some money with which to buy more properties.

He talks about obtaining private money.

Private money brokers can add value by thoroughly vetting lending opportunities and prioritizing client success over commissions.Stay Vigilant:Protect yourself and your clients by remaining vigilant and conducting comprehensive due diligence in all lending activities.

The investor would buy an ugly black box that produced money if it was the right price.

Could these waits be mitigated by going the hard/private money route?

Explore my personal favorites, Set For Life by Scott Trench or The Total Money Makeover by Dave Ramsey, for invaluable financial insights.3.

or, do you cash flow only after you've set aside money for capex and other items? 

DSCR loans are not that low and regular hard money loans will be in the double digits.