Want to get a heat gun for SMT - what should I get?
What you are asking for is specific to soldering.  It's not called a "heat gun", but a "hot air soldering station".

At a minimum, it must have a way to set the output air temperature over the normal soldering range.  This should be calibrated somehow, like a degF or degC reading, not just a warm/hot dial.  Having air flow rate control is also very useful, and I think most hot air stations have that.  Otherwise, I wouldn't worry about power too much.  That's its business, as long as it can maintain temperature at whatever flow rate you set.  I just looked at the back plate on ours, and it says 270W and has a 5A fuse.