A good house hack will allow you to live virtually for free.Use www.rentometer.com to determine rents or consult a local agent or property manager.Finally, pick up a copy of Multi-Family Millions by Dave Lindahl.Good luck!

What class of neighborhood are these properties usually in?

Eventually you can pick up the phone and the lender will gladly finance 100% and quickly once you build a good successful track record.

I'd also suggest researching small commercial multifamily properties, 5 - 30 units, and learning the benefits of that asset class.

It's all mystery to me as the listing agent don't pick up the phone.

Pick a market first so you can focus all your efforts. 

It is a D class rural neighborhood.

Unfortunately there are no credentials i can get, classes to take or a state board association.

However, inventory remains incredibly tight, with only 1.7 months of inventory in Austin and 2.4 months in the greater Austin area (a balanced market is 6.5 months).Steve Crorey, 2018 president of the Austin Board of REALTORS®, said, "In the Austin-Round Rock MSA, price classes across the board are seeing an increase in demand with a decrease in inventory. 

Ok, I guess that question was 7 questions... so let me summarize.I'm a big fan of diversifying investments over a lot of different classes, types, and areas unless I am highly confident.