for compressors its size that counts!! you need to get the one with the biggest tank you can! unless you go for a hydro vane type then your talking mega £££££s. The problem with most DIY type compressors is they dont have a large enough tank to keep up with impact tools, you need to keep letting the air pressure build back up so you get full power out of the tool hence why I say you need to get the biggest tank you can. Most are tools will state how many CFM they use so best thing to do would be look at tools and chose a compressor to suite the tools needs,