I am no expert here either. But my dad made sure to tell me beore he passed away that the AC in our shop is a 'True' 5HP AC. when it fires up, the lights flicker a little. the motor on this thing is gigantic. Its about the size of the motor in my old 93 mazda 626 with an inline 4 cylinder... I am not sure what size tank it is, but its got enough CFM to be able to run a small Brute Snadblasting pot.. Dad hard wired it into the shop, and put Air connectors all over the place so no matter where you are, you can turn around and there will be an air fitting for air.
But depending on how far down the rabbit hole you go, depends on how much money your gonna end up spending. CFM is king though, as others have stated. the higher the CFM at 90psi, the better off you are.
Also, good quality AC's are $$$$$$. Get a made in USA AC. Chinese AC's are junk.