I'm flying into ATL and arriving at 11am then picking up a rental. With no luggage to wait for, what would a reasonable time be to schedule for a date in Buckhead with a provider?

I wouldn't rush myself into getting to bucked before 2:30.  

Almost all the rental car companies are located in the same place. You will need to take the people mover next to MARTA to get to them.  I would agree 1:00 pm is a safe bet to get off the plane,  get the rental car and drive to the provider. You never know what accident will screw up Atlanta traffic, though.  
 

Since I no longer live in Atlanta, but I do visit a lot, I do this often. It should take no more than 30-45 minutes to deplane, catch the train to the rental car center and get your car on an average weekday, leaving you an hour and 15 minutes to make the twenty minute drive to Buckhead. More than enough time IMO.

BTW except for a few Sundays this summer, there is only one Braves home day game for the rest of the year, and that game is not until September, so he should be safe.