If you've got a 20+ gal tank and a 125 psi pressure switch you can run an impact easily enough for SHORT duration. It's the size of the tank you've got and the line coming out of it that counts for the impact, not the size of the pump head.
I agree that you want a better quality gun tho. Something that makes 800 plus ft-lb of torque. For moderate use a newer composite gun will work OK and be fairly cheap.