As far as compressors are concerned, get the biggest one you can afford. There's a regulator on them anyway, so if the tool needs less pressure you can always adjust it down.

I'll admit I got into air tools on the cheap - $110 compressor, oil free single cylinder, 25 litre tank. It works, it's highly portable (an advantage for me as I use it both at home and at the barn I rent elsewhere) but when doing a lot of work (suspension, parting out a car, even changing all 4 wheels...) it keeps turning on, and after relatively little work the thing needs time for recharging. Air impact wrenches tend to be used in relatively short bursts, but if you're also going to use tools that are used longer (drills, air ratchets, sanders, paint guns, nibblers, etc) you definately want a bigger tank. I'm keeping my eye on the dutch secondhand bulletin boards for a bigger compressor as well. A 50 to 100 litre tank would be nice, as would a compressor that can keep up with the air consumption of the tool. This is measured in FAD (free air delivery) on the compressor, and in consumption on the tool. If the FAD is lower than the delivery you'll eventually exhaust the tank and will have to pause for it to recharge. With my little compressor I reach that point after about 40 seconds with my air impact wrench - pressure will have fallen to the point that it seriously decreases the tool's power.

So look at the air consumption of the tool (should be listed on the box) and the FAD of the compressor, and/or just go for the biggest you can afford (and place...)

Bram