hi barry:
as long as you have 90psi at the tool, a good wrench should take off most nuts up to about 1" hex size. mine will take the lugs off my truck. i have an old ingersoll-rand 1/2" impact built before they cheapened them up. it is capable of around 150 lb/ft torque at 90 psi. make sure you have a 3/8" hose for the impact and do not run through a regulator.
i have a 5hp 60 gal single stage unit at home that i have cranked up to 140 psi max. it does everything i need to do here. it was $399 from home depot years ago. i think they are still about the same. i have a large line and connector i run a 3/4" impact from, and it works ok (175 psi would be nice). i did have to wire it 220 because that was the only voltage selection for the motor.
the least i have used, is a sears 1 hp (2 cyl cast iron head with oil bath) with 20 gal tank that made 120 psi. it still works ok, but if you have to impact the bolts for a while, you will have to wait periodically for the tank to build back up.
2 stage units are very nice, and they will give you the 175 psi you mentioned, (can be adjusted to 200 if you like to experiment) but cost goes up considerably, and you have to make sure your hoses can take line pressure. 220 volts would probably be mandatory.
stay away from oil-less compressors if you plan to put it near where you are working. they tend to be more noisy than the oil bath type.
the best compressor (IMHO) is a champion. they are not cheap and are body shop duty. i have one at our shop that went through a fire and still works perfectly after all the rubber and plastic components were replaced. they run around $1800 for a 5 hp 80 gal 2 stage.
does this help? any thing i forgot mention?
good luck,
jim