You don't need Android Auto in your car, you can download it from the Play Store
As for the launcher, it depends on which one. The more fancy effects and 3D elements it has, the more taxing it is on the processor (although most are not going to affect the battery life you're experiencing that much). Some launchers might be glitchy with certain phones (Themer, for instance, since it's widget-intensive, tends to be glitchier, but looks awesome). Most people here use Nova. It's all up to what suits your tastes.