Yep, look at the free air delivery, it's a bit like horsepower ratings in cars, quoted at the flywheel i.e engine on a dyno bench and not in a car. The horsepower at the wheels is probably 30% less on average. Don't know if that analogy helps.
Buy the biggest you can, as air tools consume loads. 3 horsepower motor is the most single phase power, but you need to wire that directly into the fuseboard. To get your 7hp you are going to need most of 12-13 cfm. You'll need all of the 50 litre tank, again go for as much as you can accomodate, space wise.
I have a 370 litre tank, with two 3hp motors, that start staggered, to give a 20cfm free air, 29.5cfm quoted by manufactuers. That's wire direct into 30 amp single phase fuse box, via a switched fuse of course. I believe that's the biggest single phase available.
I use it for car repair tools http://www.peterflynnclassiccars.com