Can someone rent an area out for an evening for 30-40 people and a band?

Yes, call city of Anderson

I'm not sure if you really need to rent. I've seen people using the stage an other venues.

The nice folks at Anderson City Hall should answer your Q's. ~ There are two areas in the park you should be able to choose from. A Gazebo and the bandstand with bleachers plus lots of beautiful lawn, both have dance floors.

Yes you can rent it from the city of Anderson. You may even be able to do it online.

Yes