The compressor that I got was from Sears: oil-less, 6 Hp and a 35 gallon tank...about $300, I think...that's plenty to run an impact, they don't use much air...die grinders and other turbine type air tools use a ton of air...
Oil-less won't last as long but they're cheaper and you don't have to filter to use it for spraying or other things that need clean air.
I wouldn't go electric on the gun itself if you want to do suspension and other heacy duty bolts. Electric is OK for lugs...but I don't think they've got enough torque for big suspension bolts that have rusted in place...
cheers,