I do extensive SMD work/rework using a Weller WMD 1A station using a hot air pencil among other things.
You need a temperature control of 300-500 degrees celcius, it depends a bit on what you are going to solder. Power is not the primary selector here.
Airflow is more tricky as, like barsmonster wrote, the crucial thing here is that you do NOT want the airspeed velocity to blow away your parts. Thus, the flow per minute is not enough by itself, you need the nozzle diameter of the hot air tool. For the Weller I use, it is about 6 mm wide and you can set the airflow between 1-10 L/min - I usually never go below 50% but at that flow, 100% will blow away stuff.
The 150 l/min you quote seems far out crazy, that sounds like a blowdryer :) Or was that in some kind of US units?
Note that in practice it is not useful to use hot air for reworking small SMT components. A soldering tweezer is much more useful here (or even a normal soldering pencil with an SMD tip). You need the hot air for desoldering chips, and for soldering chips with a ground pad.
You also NEED a microscope or benchmounted magnifying lens, but I'm sure you know this :)