If you really want to depress yourself, you actually need to size a compressor off the Free Air Delivery(FAD). This is always much lower than the rated CFM, which explains why a compressor rated at 14CFM struggles to run a tool that only needs 6!
My experience is that you'll need a compressor with at least a 2HP motor, and a 100l tank just for light use of air tools. They're expensive, heavy, bulky, noisy and expensive to run. And you'll need to ensure that its power supply is up to scratch.
Small compressors are OK for tyre-inflators, drills and painting small areas. For other tools, like grinders, die grinders, DAs, impact guns(cheap guns are fine with a decent air supply) and the like, it's best to buy electric.
I'm with Chris on the cordless impact guns; I have a tiny 18v Durofix job that barely undoes correctly tightened wheel nuts. It's real forte is winding large nuts/bolts like suspension joints, or on pullers/spring compressors. When used on my homemade extractor, it pushes out rear hubs in seconds. If you have room to swing it, then a 2foot ratchet-breaker-bar is really handy, Snap-On's is excellent(as you would expect)but 'kin expensive. Also as you would expect!