John,
I would say an air ratchet would be more than many 120V compressors could keep up with. However, it's also something that an electric impact/drill could stand in for.
I have a 1/2" impact (IR 231) that has never let me down while attached to a 4 cfm, 4 gallon compressor. With an impact I just rarely need full power. I can pull 4-5 lugnuts without the pump cycling. The hardest job I've asked this setup to do was pull an axle nut. I don't recall if the pump actually cycled during the removal. BTW, I currently have 3 compressors, the 4gal, 4 CFM hand carry, a California Air Tools 5510A (5.5 gal, 2.3 CFM, 35lbs thanks to the aluminum tank), a Westward 20 gallon, belt drive (Campell Hausfeld).
I mentioned all of this to basically say, you really don't need much to drive a 1/2" impact intermittently. A 30 gallon compressor will be nice but you can get away with less and still run the impact wrench.
If money is no object go for the 30 gallon, oil lubed, belt drive. However, if you expect to move keep weight in mind. Mine isn't too loud but it is very heavy. If you aren't going to use the thing all the time I would consider something like a California Air Tools compressor. The "1 hp" models are VERY quiet, reasonably affordable and I've found that my 5510A will run a compressor well enough to pull wheels. Like the 4 gallon unit there is enough air in the tank to pull the lugs. However, it takes 30+ seconds to refill vs 15. Still, it's really quite and at 35lbs easy to move around. It also doesn't put too much of a load on the questionable AC wiring of my garage. However, the CAT's low pressure cut in is really low.
If noise isn't a huge deal then I might consider something like the 8 gallon Harbor Freight cheap things. While oil free compressors have a reputation for being louder than oil lubed their is a lot of overlap between some of the quieter oil free models (the CAT compressor is oil free and VERY quiet) and the louder direct drive oil lubed models. Even some of the belt drives are pretty loud.
Depending on how much you think you will use it, a lower cost compressor might make more sense than a higher dollar one. Also consider how portable is important to you.