It's good to consider the floor thickness and all that. What about the compressive strength of the concrete, that's just as important. It could be 2500 psi or it could be 5000 psi, big difference. Also what is under the floor, compacted aggregate or loose fill?
As for the lift itself, I put in a 9000 lb 2 post lift. I have been very happy with it. Before we poured the floor, I dug down 3' square x 10" thick under the posts and added rebar.
