impact wrench question

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this one is for all the mechanics/do-it-yourselfers out there! i've been looking at cordless impact wrenches online for christmas and i need to know realistically how much 'break-away' torque do i need for swapping out wheels, changing springs(i understand the front strut tops don't REQUIRE them, but it makes it MUCH easier), and even the pulley swap(which i've read takes A LOT of torque). i've seen them anywhere from 117lbs to 650lbs(for real -snap-on makes one they call 'the mother') online. i know people will say 'just get an air compressor and a cheap impact wrench', but i really like the cordless option(and yes, i know it's more money). thanks for any advice.