JoJo
I agree with the previous posts. If you are concerned with multi reticle optics because of different ranges there are plenty of other good quality options. For a RDS or unmagnified Optic brands like Aimpoint, EOtech, Trijicon are the gold standard and Vortex is another great option. If you want something that can help you reach out a bit with some magnification then there are a number of good variable power scopes. I like Leupold MK6 CMRs on my mid range ARs and SCAR17 ( I just got to try a Trijicon VCOG and am liking it). There are many good 1-4x or 1-5x scopes that will not break your bank. Leuplold VXR Patrol and Vortex Viper are a couple. These have some form of a ranging reticle as well as a red dot in the center for quicker short range (unmagnified) work.
I do not beleive in "budget" optics. Knowing that your optic will hold up and just plain always work is as important as a reliable rifle to put it on. I am not saying to just buy Nightforce or US Optics or Leupold MK6 or MK8, but even if you need to save a little and wait a month or two before buying the optic then it is worth doing. Leupold makes some good quality affordable stuff particularly in their Mark AR line and VXR line. Vortex is a company that makes very very good optics and a much more comfortable price point.
Your battery life question doesnt have an easy answer. It really depends on the optic the battery is in and the type (AA, AAA, CR123). Some lithiums can be recharged which can be a $$ saver.