Two-post lifts (especially with a Benz on top) concentrate a LOT of weight on the post pad area. I'd think to be safe, you'd need footings under the posts, not mere garage concrete. Not being an engineer I can't provide the correct numbers or formulae, but if it were mine, I'd want at least a 3'x3" pad at least 12" thick with rebar or mesh embedded in each one.
If the concrete cracks under load, the lift will surely lean, and what comes next won't be pretty.
For what it's worth, I bought a much cheaper "low rise lift" (see picture
) that raises 26-27" - great for tire/brake work and body work. If I need to work for an extended time underneath, I raise the car on the lowlift and then lower it down over jackstands so if the lift fails, the car won't squish me.
M D "Doc" Nugent