Tony. What the (12 volt output etc etc) means, is simply that the output will not be accurate if run on a 12 volt battery.
The regulator chip requires the input voltage to be about 2.5 volts above the output voltage otherwise the chip will not 'regulate' correctly. Works fine on all the lower settings with 12 volts input.
That 2 Amp unit is a good choice.
HTH...
L.