I am set on the existing red going to sage green to match the existing house.  

This is your red flag!

the quote above about the 650 is red flag for sure and usually a very good predictor of a pump and dump due diligence or up front fee  scheme.... 

I would absolutely follow up with @Zach Mitchell as well who so kindly offered to look at your property and who might have a referral or two to suggest.The fact that they brought out the "we have doctors and lawyers..." line is a huge red flag to me.

We saw some small crack in the walls of a house we are interested in, but regular home inspection doesn't call out any red flags.

Red flags all over the place.

Do you really believe that statement (bolded above) is correct?

Here is the condo:http://www.zillow.com/homedetails/1761-Red-Jacket-Dr-Antioch-TN-37013/108741488_zpid/Rent comps for 2 bedrooms in the complex are anywhere from 1200-1300, but I couldn't find any rental comps for a 3 bed.

Hopefully one day you'll be big enough to actually NEED that service, and if you didn't plan for it at the beginning then whatever profit was left is either at zero or in the red.  

Are there any red flags when dealing with section 8 tenants?