So my G2 is a 2010 which is a little less aerodynamic than a 2013 G2 (Honda redesiigned the rear bumper in 2012 which knocked down the coefficient of drag by 0.01) and I also have some front bumper damage from the previous owner. I mostly got the front bumper pushed back into shape, but there might still be an aero penalty. I do have a big coroplast panel underneath the back of the car to smooth out the airflow down there.
I can get into the 60's if I drive slow on the highway, low 50's or high 40's if I go faster. If you get a 2013 or 2014 the superior aero should help your highway mileage. But you could also put
one of those panels under the back of your car to see if it helps.
Also, I've only owned this car in the winter months... as we approach summer we'll see what kind of mileage I can get. And I have the wrong tires on the front, it's supposed to use Bridgestone Ecopias, so my car might be at a significant disadvantage over the 2013 you're looking at. And the EX has aluminum wheels while mine has heavy steelies.
As for the battery, my understanding is that if you get the three software updates done by a Honda dealership then you should be fine for like 200K miles or something ridiculous. The main update is actually a recall, but when you're at the service counter ask them to look for any other software updates and apply them if they find any.
I guess what I'm saying is you should be able to do better than me with a 2013 EX in stock configuration, just get the recalls done.