Will this wall wart supply enough power?
If the drivers are good, they are constant current drivers so the coil resistance is irrelevant.  The fan draws 2W/12V = 170 mA.  So far that is 150*3 + 170 = 620 mA.  If the Arduino draws another 170 on top of that, then you're looking at 790 mA.  That leaves 210 mA of margin.  So it will probably work, but I would check on exactly how much current the motors draw to make sure you're not going to get any large transients.  The Arduino may end up drawing most of its current from USB, which would give you a bit more margin.  

Edit: I suppose the Arduino will not draw any current from the USB connection.  However, 170 mA is likely worst case and the actual draw will be significantly less.  The I/O figure will probably be far less if you're just sending logic signals as those are not actualy sourcing any significant current.