My impact gun is an IR231C $130 from a catalog, good durable tool for the money. Most compressors are made by Campbell Hausfeld under many names. I am an Amsoil dealer and I recommend using their compressor oil, it runs cooler and lasts longer than regular ND motor oil.
Most tools are rated at 4 cfm and that is conservative, impact guns will consume 12-20 cfm under load, die grinders, sanders and a few tools will run your compressor to death. This is why I suggest the synthetic oil in your compressor.
Keep your tools well oiled or they will be trash before long. I tried the lucas air tool oil and I think its good stuff, I used to use the napa brand and I had good results.
20 gal air tank will do a lot of work, I use my pancake compressor for occasional jobs at home, but it fits in the closet or the trunk of a car haha