snip

I've never used an impact wrench, but isn't the kick just or almost as
great as the force you would have to use to undo the bolt by hand?

Of course not. All the force on the hammers is transmitted to the nut
or bolt being removed. It's all in the clever application of simple
physics in the design.

I will only use the wrench to take off the lugs, and use a manual
torque wrench to properly put on the lugz.

Many pros use torque limiting extensions on lug nuts or bolts.
Personally, I set the the air control on my impact to a low number,
check the resulting torque with my torque wrench, then finish the set.
With a well regulated shop air supply, the impact wrench setting will
seldom need changing and over-torquing doesn't happen. This is a
concept that takes a while for an apprentice mechanic to understand
and some places like Whoopee Lube never get it.

I have had lug nuts that were on so tight, I had to stand on the
wrench to get them off. Would an impact wrench help in that
situation?

It's the only thing that will do it for you. Even the most carefully
torqued nuts and bolts can build up corrosion enough to act like they
are welded on. reiterating, don't send a boy to do a man's job. Get a
tool with the guts to do the job. HTH

Joe