Why bother with the lowrise lift?
Having a 2 and 4 post lift, If I were to do it again, I would do a 1 post storage, like this:
https://aclifts.com/lifts/m-1-single-post-car-lift/
and a 2 post.
As to the excavation, think seriously about what Java said. On a slab that large we pour 4-6 pads on virgin ground(when we pour footing), and dry stack open core cement blocks to grade. When you pour the floor, they will fill in with concrete. I would aim to place them in the vicinity of the lifts.
I also made a "grade beam" when I poured my current garage, when we levelled the stone I left a channel 4" deeper across the floor, in the vicinity of the 2 post lift, so the concrete was 8" thick in this area. Extra rebar in channel, make sure contractor supports it in the concrete, not lying on the bottom.