I was thinking about putting down 10-20% in cash and then seller financing the rest at 5.0-6.0% interest rate over 30 years. 

You'll want to understand what kind or notices and documents you need to get executed to either up their rent or turn the unit.Solid long term tenants are worth their weight in gold in my book so if theres a way to up the rent and keep them happy I's say thats the best route personally.

Verifying last 2 years of rental history and income extremely important to find the “best of the worst”.

FasterFunds Lending, a hard money lending who focuses on the Saint Louis Market has a unique take on hard money lending that I would think should be more popular.
1. No appraisal fees. They get in their cars and go s...

All the best,Josh Messinger

Rates tend to be higher, since these aren't "conventional" loans that are going to be bought by Fannie or Freddie. 

If not, schedule that activity for some time later in the day/week after you've taken care of the income-producing activities that will help you grow your business.Lastly, I'd recommend reading some of the David Greene books for real estate agents in the BP bookstore as well as Hyper local Hyper fast agent by Dan Lesniak.All the best!

What is the minimum interest rate we can charge for the seller financing without causing any red flag with the authorities?

The goal is not to cash flow, and it never was.a lot of new investors run the numbers on a house hack and get discouraged, because the rent they'd take in wouldn't totally offset the mortgage. but the goal was never to cash flow positive - that was just a feature of the market several years ago when interest rates were so low.

My failures are my own are my best (and most expensive) lessons.