Using a LiPo battery or different battery for outside project
I presume this is a somewhat sealed project, if you have room for a lead acid then have a look into LiFe batteries, they are similar to LiPo, but without most of the safety concerns of LiPo

Lead acid will have some small off gassing, and the hydrogen can cause solder joints to become brittle, this is part of my reason for steering away from that, if you have some mitigation towards that then lead acid should be fine.