There is always the Costar awards for volume etc that you can look through to find the people doing the most business. 

I am new to the real estate investment business, and I inherited 3 properties that are currently in my wife, and I trust. 

This is far from an easy business.

Bottom line -- how could someone build a *profitable* real estate investment business with little cash of their own and without rich family/friends?

The property is located in the Midwest and me and my business partner have ran the comps, numbers and called numerous property managers, handymen, inspectors and even asked locals on reddit to get a better understanding of the rental calculations and local biases in the area. 

I could probably help out a bit, but I've been super busy. 

For our STR we have a few different types of loans but I would always go with the 30 year fixed because you never know life gets busy and in 5 years a lot can change. 

I own a 3-unit multi-family with a business partner.

If the property is being sold for 400k (and I can afford the 20% down) but another 800k min needed for a complete renovation (my personal rough estimate), can I take out a traditional business loan to cover the rest, if I were to purchase in my LLC?

There comes a time that you just have to make a business decision about raising rents because owning real estate is expensive.