you don't actually need an impact. If you have a large hammer and a flat tipped punch you can break the screws loose. They will be frozen in there if the car is more than a year or two old... So don't just try to turn the screws with only a screwdriver.
Put the flat part of the punch against the screw and whack the sh!t out of it.
This trick will work for allen head screws that are stuck too. The impact causes whatever is stuck to come loose.
Although an impact screwdriver is professional way to do it.